My Little Sister Is the Best F*p Material

Volume 8 Chapter 06: Someone Else’s Boyfriend

“Sorry to keep you waiting.”

Waiting outside the park’s restroom, I saw Yamamoto-san return, her eyes still slightly puffy but otherwise back to normal. Most of the planned events were over, leaving only overnight guests. The area was now fully dark.

“Glad you’re calmer.”

I could only manage something neutral. We’d take the train back, transfer to a bus, and that would end our summer—at least as more than friends.

“Um… want to stay at my place tonight?”

She fidgeted, tilting her head with an awkward smile.

It was unexpected. After such a clear end, I thought she’d step back cleanly. But her tone wasn’t desperate to hold onto me; it felt purposeful, like she had a reason.

“Maybe…”

I had a half-certain hunch.

“Did Miyu say something?”

When I asked, she nodded quietly. I’d learned by now that surprises like this usually involved Miyu. It probably stemmed from the day she came to take care of me.

“If you’re okay with it, I don’t mind.”

That’s what I felt, so I agreed without overthinking.

“Thanks, Sotomichi-kun. Let’s head back.”

Leaving the quiet park, we got lucky with side-by-side train seats but sat in silence, unable to find a conversational spark.

Feeling the seat’s vibrations, I stared blankly at the message I sent Miyu:

“It’s over. Heading to Yamamoto-san’s now.”

Was this ending happy for her? Sure, settling things clearly might be better than lingering regrets. But Miyu said she wouldn’t trample Yamamoto-san’s feelings, yet this breakup felt like just that.

On the way back, despite returning to “friends,” we didn’t talk like we used to. She seemed lively, but I didn’t know what to say. After getting off at our station and boarding the late-night bus, she finally spoke.

“School starts tomorrow, huh?”

No eye contact. Summer break was over; classes would start after the opening ceremony, followed by exam prep.

“Yeah. Feels weird.”

This summer, Yamamoto-san and I had so much happen. Beyond that, my unmentionable moments with Miyu and her friends made it an intense season.

Those memories would fade into routine once school began. We’re classmates, and on paper, nothing changed from before summer. But I wasn’t confident I could talk to her like before when we met at school.

Would we drift apart? As the one who rejected her, I had no right to say it, but it felt deeply lonely.

“Ha… I was gonna tell everyone I got a boyfriend if things worked out with you.”

“Seriously? I’d be dead meat.”

“Nah, it’d be fine. Time would’ve sorted it.”

Imagining us as a couple, envied or celebrated at school—a vivid future now reduced to fleeting fantasy. Did she dream of being my girlfriend before this date? That thought tightened my chest.

Our light conversation fizzled. On the bus, I was too scared to look at her.

No reply from Miyu. She might’ve seen the notification or was already asleep. While I wondered, we reached her stop. We walked close enough for our arms to brush but didn’t hold hands until we reached her place.

“Come on in.”

“Feels like it’s been a while.”

“Yeah.”

This was where we nurtured our love. Inviting me here must stir painful memories for her, so why?

“I’ve got one last request, or rather, something unfinished.”

“Unfinished?”

In the living room, as I hesitated where to sit, she said it.

“I need to give you a letter.”

“Oh…”

It clicked. The sealed envelope I found here, which she called a breakup letter. Given by Miyu, it was exactly that—a letter to make her say goodbye.

She pulled it from the mattress gap and opened it. Unfolding an A4 sheet, she handed it to me.

“Thanks for indulging my selfishness. I’m grateful to you and Miyu-chan. …So, um, I want to fulfill what’s written here…”

Suspicious of her sudden shyness, I read it. It was a pledge: she’d give up her love for me under certain conditions, signed by what looked like Miyu’s handwriting.

“More like, neither of us has a choice…”

“What… this is from Miyu!?”

The condition boiled down to: “Have as much sex as you want with him before breaking up.”

This promise, meant to let her part without regrets, was likely intended for the day I rejected her. When I told Miyu in the bath that I hadn’t slept with Yamamoto-san, her shock made sense now—this was why.

“So that’s why you avoided going all the way.”

The promise was delayed, and she stopped choosing sex with me. The hotel incident was her desperate compromise, only possible because Miyu allowed me to cum. Otherwise, she’d have given up.

“It was supposed to be given earlier. Its validity’s shaky now, and I didn’t have to keep it.”

I’d heard this at the hotel. This was the backstory. Even as the pledge’s terms blurred and Miyu’s scheme clouded her judgment, she clung to its form.

“But at the park, you said no limits.”

“I thought it’d end one way or another today. If you wanted me, the pledge wouldn’t matter.”

But I didn’t become hers, leaving only this fragile promise. Miyu agreed to this sex. The signed paper ensured I didn’t need to check with her.

“Got it. And no choice means?”

I’d heard but sidestepped it, picturing Miyu’s devilish whisper.

“If I accept defeat, no matter the outcome, I have to follow the letter… Miyu said.”

She foresaw even this. Total devil.

But the cruelty of this end depends on me.

“Let’s do it. I want to, too.”

I wanted to hold on. I couldn’t accept this half-hearted breakup. That’s how strongly I felt.

“Really!? Thank you…”

She added a fulfillment signature to the pledge and sealed it. After crying and reaching closure, I couldn’t brush this off carelessly.

I’d fight to the end my way. Miyu probably saw through that, too.

“Sotomichi-kun.”

After putting the letter away, she stood straight, facing me, a trace of hesitation in her fingers.

“I love you. Now… probably forever.”

It was precious to us but now a meaningless confession.

“I’ll always love you, too.”

I hesitated but chose honesty. Lying to her, who’d have to accept reality, felt pointless. I’d be truthful tonight.

I realized now how much this letter tormented her. Miyu told me to date her as she wished, allow sex, and act like a boyfriend. I worried we’d get too close, maybe become real lovers, but the reality was the opposite.

No matter how much fun we had, returning to her room brought Yamamoto-san back to reality, courtesy of that letter. In the quiet solitude, the closer we got to acting like lovers, the harsher the pain when clarity hit. It stripped her of reason, leaving her no choice but to take Miyu’s challenge. Torn between supporting my bond with Miyu and wanting to be my number one, her self-contradiction led to today. That was Miyu’s magic spell on her.

“Shower before sex?”

“Yeah, I’m pretty sweaty.”

We moved to the bathroom, stripping casually. Jumping in together felt natural since we were about to have sex—good for easing into it.

Yamamoto-san’s curvy body had perky nipples, and she discreetly tossed her panties into the washing machine. Her arousal was likely intense.

“Ugh… don’t stare like that…”

She covered herself, ashamed of her excitement. I knew that mix of guilt and lust from past experiences. Having just been relieved in the restroom, I was calm, but she’d been pent-up all day. Even if Miyu’s orders forced this, sex with someone she loved was her favorite thing—she couldn’t not be turned on.

“Um…”

She looked at me pleadingly.

“Would you be mad if I kissed you?”

Sex was allowed, so a kiss should be fine, right? For most couples, maybe. But for me, where sex often comes before intimacy, her question made sense.

“I won’t be mad. If it’s hard for you to initiate, I’ll find the right moment too.”

“Really? That’s sweet.”

Her ladylike demeanor lit up at my offer.

“So, about that…”

I mumbled apologetically—it wasn’t about kissing—and dropped my pants. Despite worrying if I could perform, my cock was rock-hard. Miyu always said it’s flattering when someone’s aroused by you, and seeing Yamamoto-san’s excitement got me going too.

Her gaze dropped to my erection, and she smiled slightly.

“Should I kiss this hard thing?”

“N-no, I mean… a regular blowjob… but if you’re offering a kiss, I’d like that too.”

“Hehe, got it.”

She knelt, peppering my cock with kisses before taking it in her mouth. Unlike the urgent restroom blowjob, she licked gently, savoring the lingering cum. No hesitation, even though we’d shower soon.

“Suck… chuu… slurp… smack…”

“Ugh, ahh… feels good…”

Her restrained approach was irresistible. A girl sucking me off so earnestly filled my heart.

“Lick… chuu… pop… Seems like you’re doing great?”

“Yeah, pretty good. I’ll hold off cumming until the main event.”

“Mhm.”

She went to suck again, licked the tip, and pulled back.

“Want a paizuri to commemorate?”

“Uh, sure.”

Commemorate what, I wasn’t sure.

She lifted her breasts, enveloping my cock.

“Nn… suck… kiss…”

“Ah, ahh… damn, that’s intense…”

Her bouncing breasts wrapped around me, the tip peeking out, met with shallow, lewd sucks.

“Ugh… too good… ah, ahh…”

“Hehe… slurp… suck… smack… Your precum’s delicious… chuu… slurp…”

“Kuh, ugh…!”

This was bad—I might cum. I’m confident in my stamina, but a moment’s sage mode could ruin what’s next.

“Suck, chuu… pop…”

“Stop, stop! I’m gonna cum.”

“Hm? Really? Gotta cool it down then.”

She freed my cock from her mouth and breasts, blowing gently on it. My cock twitched violently.

“Ah, ugh…! That’s bad too, sorry!”

“Even this blow-cool trick’s from Miyu?”

“…Yeah.”

“Such a perverted sibling duo.”

My instant loss of control tied to Miyu’s tricks betrayed me. What a troublesome body.

“Shower time, then.”

“Together, right?”

“Hm? Yeah, hehe.”

Her sly smile confirmed it. Showering together meant washing each other, an unspoken agreement.

In the bathroom, she went first. I lathered soap with a net, spreading it over her body. Every inch was soft and erotic.

“Your hard-on’s poking my butt.”

“I think I can last until the bed.”

“Yay.”

To keep my arousal, I kneaded her massive breasts while pressing my cock against her ass. Even though penetration wasn’t possible standing, the soapy slipperiness sparked raw insertion thoughts, sending my brain into overdrive. Like getting turned on by my sister, this thrill-seeking body needed taming. Miyu wouldn’t refuse if I wanted kids.

“We’re using a condom in bed, right?”

Just checking. Even if the park date had worked out, she planned on condoms, and that shouldn’t change.

“Yeah. The pill’s for extra safety. Plus, I’ve never done it raw… cumming inside’s still a bit scary.”

“Got it.”

Taking responsibility and fearing pregnancy’s unknown were separate. Even on the pill, cumming inside was daunting. Normal.

Then again, I’ve been going raw with my sister…

I’ll keep that quiet unless asked.

“Your turn.”

“Go easy on me.”

“Don’t worry, I’ll make sure you don’t cum.”

She re-lathered the soap, spreading it with her hands across my unwashed body, using every curve to clean me.

“Ugh, fu… Your body’s a weapon for this kind of play…”

“Free and unlimited for you.”

“Feels too good, I’m almost sorry… ugh, ah…!”

She teased sensitive spots like my neck and lower abdomen with her hands, while her breasts cleaned the rest. Feeling her hard nipples on my back, my neglected cock twitched. Then, she slid beside me, trapping my arm and leg between her cleavage and thighs, sliding up and down.

“Wait, that’s too much, I’m gonna cum…!”

“What? I’m not even touching your erection.”

“Still, your breasts are too intense…”

“Oh my. Guess we’ll rinse off then. I kinda wanted to see you cum just from my breasts squeezing your arm.”

“Please, have mercy.”

Yamamoto-san stopped her tantalizing movements, freeing my repeatedly edged cock from stimulation. She turned on the shower, washing away the soap from both of us.

“This is fun, Sotomichi-kun.”

“Yeah.”

Even after the soap was gone, she let the water run. Without speaking, the shower’s sound filled the bathroom. I couldn’t see her behind me, but I knew she wasn’t just rinsing herself.

Eventually, she turned off the water, hanging the showerhead, droplets dripping from her hair.

A soft ton hit my back. The lively bathroom fell silent. She was crying.

“Ugh… why did it turn out like this…?”

She wanted to lose herself in joy, but that was no longer easy. “Should we stop?” flickered in her mind, her pain evident. Despite Miyu’s orders, I could end this night by refusing, but her fierce embrace pushed us to keep going a bit longer.

“Nn…! Alright! Let’s do it!”

“You sure you’re okay with that energy?”

“Yeah! Crying it out really helped. Even if I get down again, I’ll bounce back, so just watch over me.”

“Got it.”

We left the bathroom, dried off, and moved to the bed without dressing. Once sex started, we’d dive back into that affectionate world.

Believing that, I had her blow me on her knees, then put on a condom. She lay back, and I entered her missionary-style—a typical lovers’ act. I pressed my erection against her entrance.

“Here I go, Yamamoto-san.”

“Come. Make me feel it.”

I slid into her, moving slowly at first. It helped her feel my shaft, but… after the shower’s emotional moment, she wasn’t as wet as expected. Going harder might hurt her.

“Feel good?”

“Yeah. Really good. Keep going.”

Her words were empty. She had no other answer. Admitting it didn’t feel good would taint her final sex memory with sadness.

“Sotomichi-kun, please. Kiss me. It’ll help me get wet.”

She knew her state, reaching out to invite me. She’d wanted me to initiate, but her desperation made her beg. She was at her limit.

I wrapped my arms around her neck, pressing close, kissing deeply with tongue. A light kiss wouldn’t suffice now. Her kisses grew intense, and we pulled each other closer, as if to erase any gap.

“Nn… chuu… haa… mm… chupa…! Sotomichi-kun… jurur, chuu… chupa… faa… haa…!”

“Haa, nn… Yamamoto-san… mm, churu, jupp…!”

We kissed hungrily, seeking to fill an emotional void, chasing intensity over pleasure, drowning out intrusive thoughts.

But it filled nothing. As our bodies grew used to the fervor, sadness crept in. She pulled away, her face tearful, clutching my arm.

“Sotomichi-kun… thrust deeper… harder…!”

At her plea, I pounded with all my strength. Rough, uncaring sex—if it could make her forget her pain briefly, I drove my hard cock into her repeatedly. But the usual wet guchu sounds of her arousal never came.

We both realized the act’s futility. Knowing it, I stopped moving. Only our heavy breathing and the throbbing pain in my temples offered relief.

“Why… why am I so selfish… it should be over… but still…”

Covering her tear-soaked face, she blamed herself. She couldn’t enjoy it because of regret—regret from not letting go of me.

The sex after the letter was meant to end her love. She had to keep that promise, no matter what. But this sex being her last was more painful than death.

“Please… I beg you…”

She spoke the forbidden words.

“I’m fine being second… even just a fuckbuddy… just let me stay with you…”

A wish that nullified today’s efforts. Understanding her, I couldn’t comfort or reason with her.

Her eyes lost their resolve to make it happen, but her lingering feelings were buried deep within.

“Sotomichi-kun.”

Her voice trembled.

“Kiss me.”

With a tearful smile, it was all she could ask for—the last fragment of her hope.

I cupped her cheeks, wiping her cold, fresh tears with my thumbs. Kissing her felt like ending everything, so I fought the urge to comfort her, searching for words.

No matter how I thought, I couldn’t save her. Taking responsibility without the ability only led to regret. I was powerless.

“How dare you act so selfish after screwing someone else’s boyfriend.”

A sharp voice cut through. From the door, an impossible voice—one that shouldn’t be here—reached us clearly.

“Wha—”

Turning, I saw my sister, Miyu, standing with her usual scowl, hand on hip. Her tight T-shirt emphasized her chest, and a rare short skirt showed off her toned thighs.

“M-Miyu!? W-Why are you here!? The door was locked…!”

Yamamoto-san, shocked, her tears drying, hadn’t noticed Miyu’s entry mid-act. But if Miyu had been hiding, Yamamoto-san’s sharp senses would’ve caught her.

“The pledge had two unwritten conditions.”

Miyu approached, explaining. One was Yamamoto-san taking the pill. The second…

“Giving me a spare key.”

She placed an unfamiliar key on the table.

“Kanade-san could lock Onii-chan in her room or force him to cum inside if she wanted.”

Miyu’s words made Yamamoto-san react.

“I wouldn’t!”

“It’s about whether you could.”

“Well… if it’s about capability… yeah…”

Right. She could easily restrain or arouse me. Even if I need Miyu to cum, there are workarounds, like that edited photo.

“Let’s reset. Get dressed first.”

Still connected, I tried to pull out, but Miyu stopped me.

“No need.”

Here we go with her nonsense.

“You two were really into that kissing earlier.”

Her tone had an edge.

“Kissing’s that special, huh?”

She’d heard Yamamoto-san’s plea. How long was she here?

“With Yamamoto-san, it’s… well…”

It was our first and last, meant to be special beyond sex.

“Hmm.”

Miyu glanced at our joined bodies. My erection held, still inside her. I’d been seen mid-sex with Yuka before, but this felt awkward.

“Feels good?”

“Well… kinda, but right now…”

Her dryness made the pleasure worlds apart from usual.

Yamamoto-san looked apologetic. I wished I could’ve given her sex that felt truly good.

“What about before?”

“Of course, those were amazing. Unmatched.”

Every time with Yamamoto-san was blissful. Her selfless devotion to my pleasure reached a level even Miyu couldn’t touch, I could say with certainty.

Especially when she was serious. In the karaoke booth, the love hotel, the restroom—her dedication to making me cum, infused with lover-like affection.

Unmatched—so much that it slipped out. Sex with her was that good.

“So…”

Miyu drew closer.

“Which feels better, me or her?”

The question sent a chill down my spine. It was outrageous, asked in front of a grieving Yamamoto-san.

But I’d misunderstood—Miyu said she’d be thorough. If she said thorough, just making Yamamoto-san cry wouldn’t cut it.

“Kiss the one that feels better.”

The worst possible command. Yet, it was karmic for Yamamoto-san. She’d asked the same during a call with Miyu while I was having sex with her in this very room.

“They’re… about the same…”

I didn’t mean to lie. But as a hopeless siscon and lolicon, I had a clear answer. For Yamamoto-san, hearing it would be worse than being violated.

“If I had to choose… maybe Miyu…”

I tried to dodge with a safe answer.

“You said kiss the one that feels better, right?”

No escape. I had to kiss Miyu to prove her sex felt better. It was cruel beyond measure, but I could do it. My body had been conditioned to compare their sex.

“Sotomichi-kun…? You wouldn’t, not now…”

Yamamoto-san hoped I’d kiss her, even if it was a lie. It’s what she wanted.

But reality was harsh. No pleasure surpassed sex with my sister. That truth wouldn’t change, even if the world flipped.

“Sorry, Yamamoto-san.”

With that, I kissed Miyu.

“Nn…”

My lower half still connected to Yamamoto-san.

“Hamu… chuu… juchu… chupa… fa… Onii-chan…”

“Miyu… nn…”

I kept kissing her. I didn’t see Yamamoto-san’s expression—truly didn’t. But trouble came next.

“Nn, fwe!? S-Sotomichi-kun… something’s… inside… ah…!”

Yamamoto-san, clutching our joined bodies, was overwhelmed by sudden pleasure.

“It’s… big…! No, not like this… ah, an…!”

Kissing Miyu made my cock surge. It had softened in her dry vagina, but now it swelled rapidly inside her.

“Wait, wait, what’s… hyu, ah, aaah…! No, stop…!!”

My cock, triggered by Miyu, made her feel it alone. As we kissed, Miyu glanced at her, parting her lips briefly.

“Onii-chan, the contract with Kanade-san’s still active. Don’t leave her hanging.”

Miyu, ever dutiful, ensured her promises were kept. I thought continuing with Yamamoto-san like this was wrong, but her vagina, now wet from my cock hitting her sensitive spots, felt too good. My hips moved on their own.

“No, no, stop… I don’t want this… ahh, guah, nnaah…! Hii, faa, ah, ah… please, stop…!”

Yamamoto-san, possibly already cumming, couldn’t accept or reject the pleasure. Forced sex with Miyu’s orchestration was humiliating, yet the sensation was undeniably blissful.

“Ugh… why… why… I’m the one with Sotomichi-kun… ah, faa, aaah… but…!”

Watching us kiss passionately, a result of comparing her sex to Miyu’s, even her pleasure stemmed from my sister.

It was like being cuckolded, despite her being the one having sex. She’d felt this before, in this room—a second time being cuckolded while being fucked.

“Ugh, crap, I might… cum…”

Separately, I was close. Since Yuka, my body got off on being forced.

“Just let it out.”

“That’s too much…”

It disregarded Yamamoto-san too much. She heard us.

“No… not that…! Sotomichi-kun… ahh, aaah… look at me…!”

She tried to stop me from cumming. But with Miyu’s go-ahead, I thrust pan, pan—

“Hiii, ahh… nguah, ah, aaah, anaa… fa… aaaaah…!!”

Yamamoto-san moaned in pleasure, unable to form words.

“Onii-chan, don’t hold back.”

Urged by Miyu, I kissed her again. In her forceful, loving kiss, I came easily, byuku, byuku.

“Haa… haa… fa… ah…”