My Little Sister Is the Best F*p Material - Volume 8 Chapter 01: Miyu’s (First-Year) Gym Uniform

Volume 8 Chapter 01: Miyu’s (First-Year) Gym Uniform

A man’s thought process shifts depending on how much sexual tension he’s carrying. For someone like me, who ejaculates daily, even a single day of abstinence fills my mind with erotic fantasies by the next. The real issue is how, like a drunk losing inhibitions, horniness dulls my aversion to perverse acts. Fortunately, my sister’s libido seems to sync with mine, so she doesn’t push me away—but she’s quite the pervert herself (though she’d never admit it). Reflecting on my own mistakes later always stings with regret.

“…Why are you staring at me, Onii-chan?”

Miyu and I were in the laundry room at home. After breakfast, while I was washing my face, she came in to load the washing machine, holding clothes she was about to toss in.

“No reason—just wondering why you’re washing your gym uniform.”

She held a fresh, blue tracksuit set. Since Miyu wasn’t in any clubs and it was summer break, there was no reason for her to use her school gym clothes, so washing them puzzled me.

“Did you… use it?”

“What’s with that question? The school year’s starting soon, so I’m just washing out the closet smell.”

“Oh, okay. That’s fine… though it’d be okay if you did use it.”

My horny brain was the problem here. Imagining my sister using her gym uniform for cosplay masturbation was a low point for a brother.

“As if I’d get turned on by clothes I wear regularly,” she said, giving me a cold stare.

She’s all about logic, this sister.

“So, you’re washing it?”

“That’s why I’m here.”

She shot me a look, questioning my obvious remark.

“Well, it feels… kinda wasteful.”

“Your twisted thoughts are clear, but explain.”

“So, like…”

Miyu’s a third-year now, meaning she’s worn that uniform for over two years of gym classes. That’s a lot of time in it.

“Maybe we could… use it a bit before washing?”

“I washed it before summer break.”

“Right…”

“And you’re mistaken—this one’s only been worn a few times.”

“S-Seriously?”

Damn, right. Yuka borrowed it without permission, so Miyu replaced it with a new one. Yuka’s meddling ruined a potential treasure.

“And we replace them yearly anyway.”

“Why would you…?”

“I didn’t just grow in the chest these past two years—my height’s up too, obviously.”

I knew that, I did. Facing the reality head-on was just tough.

“By the way, you don’t keep last year’s, do you?”

Miyu froze for a split second, a tell only I’d catch, then answered.

“Nope. Even if I did, what would you do?”

Her flat tone was too obvious. There’s a chance.

“Nothing. Just… look.”

“Liar. You want me in a tight gym uniform to sniff it, don’t you?”

“Well, yeah, but…”

“No way. Absolutely not.”

She stuffed the uniform deep in the machine and slammed the door. Her strong resistance was telling—denying it so hard meant it was a real possibility for her.

“Please.”

“No.”

“Fine, just a photo then…!”

“What stubbornness for using your sister as fap material.”

She stormed out, her parting words and actions nostalgic.

“Don’t look in my closet, or I’ll be mad.”

Practically admitting she kept her old uniform. But with that resistance, I slunk back to my room, deflated. Then, Miyu came to me.

“If you’re pent-up, just use me.”

“I was gonna ask. Just wanted the gym uniform.”

“It’s a spare, not for sex.”

She sat on my bed, dangling her bare legs. A girl in my room—what a view.

My theory: she uses the uniform for masturbation. Miyu, who loves using herself as material, saying she wouldn’t get off on her own clothes was pure sophistry.

“Let me be honest.”

“You’ve been pretty honest.”

“I think you use the gym uniform for solo play.”

“Think what you want.”

“If I’m right, you’ve got your first-year uniform too.”

“…”

Talkative Miyu went silent. If it’s just a spare, keeping an ill-fitting one is pointless—clutter in her packed closet. But if it’s precious, she’s holding onto it to wear her younger self’s clothes, masturbating to that image in the mirror.

If I checked her closet and found it, she’d be caught. That’s why she couldn’t dodge with a clever reply.

“You said you’d respect my hobbies.”

She finally gave in, her unchanging voice and expression part of her charm.

“I do. I get you want to do it privately sometimes.”

She’s less opposed to clothed sex with me now, but cosplay is partly for her own pleasure, and I don’t want to take that away.

“But I can’t suppress this curiosity, right?”

“Pretty grandiose for just wanting to cum.”

She’s right, always truthful, never lying.

“…Fine, whatever.”

Her killer phrase, casual but exclusive to me, hinting she’s up for some fun.

“I haven’t used it yet, just so you know.”

She warned me as we moved rooms. Claiming a year-old uniform wasn’t used—Japanese is wild.

Opening her closet, she pulled out a gym bag from the back, loosening its strings. Out came a clearly worn white uniform, and just thinking it was her first-year one made holding back an erection tough.

“Might not be as thrilling as you think.”

“I’m already excited, so it’s probably fine.”

“That weird confidence is your best trait.”

She twirled her finger, signaling me to turn around as she changed. Even as lovers, she’s the only girl who won’t let me watch. Not wanting her to change her mind, I faced the wall. Nothing trumped seeing her in that uniform.

The sound of clothes rustling—or hitting the floor—behind me was so erotic, especially since it was prim-and-proper Miyu.

“Wow… so small…”

Her comment after changing raised my expectations. Her growth must’ve been more than imagined. I couldn’t wait to see her tight uniform. Too bad I was already hard.

“Can I look now?”

“Uh, y-yeah.”

Turning at her permission, I saw Miyu tugging the hem to cover her navel, her gym uniform accentuating my erection. The shorts, short for her height, showed off her legs like hot pants. The tight fabric outlined her breasts perfectly—no way the boys at school could see this.

The underwear visible through the fabric was striking, but the T-shirt’s wrinkles were the sexiest. Her chest formed a deep valley, the fabric stretched taut, creating tiny creases from her high peaks.

Most importantly, she’d worn this for a full year two and a half years ago, sweating in it. The fabric, warmed by her body, faintly carried her sweat’s scent.

“Can I sniff it?”

“What you’re smelling is an illusion—just detergent. You’re sane, right?”

“Sane enough. But lasting till I cum? Not so much.”

“Too quick for just my sweat’s smell.”

While bantering, she tied her hair into a ponytail, likely her usual gym style. Her classmates must be jealous, but they can’t even get close, let alone smell her. As her brother, I’m the winner. If I could just clear the remaining hurdles, I’d be unstoppable.

“Can you jump a bit, like with a jump rope? I wanna see your boobs bounce.”

“You don’t need to spell out your goal.”

“You’ll do it?”

“It’s within expectations.”

Stepping back, she cautioned, “Don’t get your hopes up,” then hopped lightly. Her breasts bounced, the fabric shifting, undeniably erotic—but somehow lacking.

Miyu seemed to sense my slight disappointment, murmuring, “This might be better,” as she switched to stretching, extending her arms. The way her muscles, usually hidden, moved under the skin-tight gym uniform was oddly erotic, given how much skin it already exposed. Glancing at the mirror, arching her back, she earnestly tried to strike sexy poses, her effort both adorable and endearing.

“If you filmed radio calisthenics in that outfit, you’d easily make a million yen streaming it.”

“Don’t underestimate my worth at just a million.”

Miyu was confident in her body. Active in Haruka’s photography club, where sums far exceeded a million, that amount was pocket change to her.

“Phew,” she exhaled, catching her breath. Stretching was still exercise.

“Body’s nicely warmed up.”

“I’m scared I already know what you’re gonna say next.”

“Should I not say it?”

“You don’t have to, but, like I keep saying, it’s just detergent smell, okay?”

I got her point. Modern laundry tech leaves no trace of sweat’s essence. Yet, this gym uniform was special. An undeniable truth: first-year Miyu wore it, sweating through it repeatedly, soaking it with her body’s moisture. That very garment was here, worn by the same person. I had to smell it.

“Miyu, don’t run. Just bear with it a little.”

“Nn… if it tickles, I’ll push you off… ha, hyan…!”

Grabbing her shoulders firmly, I sniffed from her nape to her collarbone. The pheromones already seeping out were more than enough to drive me wild, the rich scent of her curves carrying a warm heat from her cleavage. She wasn’t sweating from stretching—her body heat rose from the thrill of wearing her old uniform. Embarrassed at being found out, her temperature spiked further.

“Stop acting like such a pervert… ffu, nn… don’t you have any shame…?”

“You’re making me do this… smelling so erotic…”

“It’s not my fault… nn, ah, no, not there…!”

From her neck to her cleavage, armpits to stomach, my attention inevitably shifted downward. Inhaling deeply at the seam of her gym shorts, she twitched as if receiving cunnilingus, grabbing my head.

“Not there… nn, no, don’t… don’t smell it…!”

“It’s strongest here… your scent, Miyu… haa… haa… this is why I love you…”

“Uuu… ffu, haa, haa… nn… stop saying weird stuff…! Haan, any more and… hann… nn…!”

Desperate to smell her directly, I pressed my nose in, accidentally stimulating her clitoris, making her cum. I’d hoped to see her climax from the shame of having her scent sniffed, but maybe that was too much to ask.

“You idiot, Onii-chan…!”

Slumped on the floor in a girly pose, glaring with bloodshot eyes, she stirred a nostalgic joy in me. I reaffirmed that lust was toxic. As lovers, I should avoid such shameless play.

“Miyu, can I make one more naughty request?”

“Still no hesitation using your sister to get off, huh?”

It’s because it makes you happy. Like a dog trained to obey for treats, I’m a brother conditioned by my sister. The only difference is my drive comes from lust.

“How much naughtier can this get?”

“Just go no-panties, no-bra. That’s all.”

“Your words and pride are so light…”

She shot me a contemptuous look—a reward in itself—then, gripping the bed, stood half-bent, flaunting her insanely erotic cleavage, pushing me to the edge of climax before pointing to the wall again.

All she said was “Nn,” confirming she’d go braless as requested. Her indulgence as a sister nearly made me cum on the spot. I had to at least see her nipples through that white T-shirt.

“Done.”

Briefly, she signaled she’d changed. Turning, I saw her discarded panties on the floor, one hand covering her nipples, the other clutching her bra strap. Outwardly the same, but inside, she was like a seductress, subtly signaling it. She knew exactly how to turn me on—resisting ejaculation was brutal.

“Now jump again like that.”

“No way!”

“Why not? You did it before.”

“Give me a second to prepare…!”

“This kind of pervert play is easier to blur through with momentum. Thinking too much forces you to face what you’re doing.”

“Ugh… why are you so eloquent only now…!”

But it was the truth. Dropping her arms, she arched her back dramatically, emphasizing her breasts, then did short squats to make them bounce. Unlike the bra-contained, ball-like motion before, her breasts now jiggled with a soft, vibrant bounce. The darkened tips—her nipples—were an outrageous sight.

Her sensitive nipples rubbing against the cotton T-shirt made her wince slightly, adding to the eroticism. Synesthesia seemed to link sight and touch; my cock, feeling like it was being paizuri’d in my pants, forced me to free it to avoid cumming.

“Can’t hold back. I’m sniffing.”

“Just erase my memory with it…”

“Leave it to Onii-chan.”

Laying her on the bed, I buried my face in the gym uniform, now vividly outlining her breasts. I’d fuck her, cum inside, and end this play.

Since she’d gone this far, I had to prioritize her mental safety. Her humiliation from being caught using her old uniform for masturbation was bad enough, but jumping braless, her nipples stimulated, soaked her shorts like she’d wet herself—she couldn’t stay sane.

Inhaling deeply in her sweaty cleavage, filling my lungs with her scent, I slid my cock into her through the gap in her shorts. A vulgar, squelching sound followed.

“Hya… agu, ah… hiaa…!!”

She came instantly as I pushed in. She’d likely climaxed lightly a few times already, so I won’t dwell on the speed.

Propping myself up to watch her disheveled in the uniform, I savored my cock sliding into her shorts’ hem. I wanted to suck her nipples through the shirt or, if it were post-gym and sweat-soaked, drain it dry. But to preserve it for her solo use, I restrained myself.

“Miyu… Miyu… haa… you’re too erotic, I’m so pent-up… fuu, haa… I’m cumming all at once…!”

“Ngu, nn, haa, aah… Onii-chan… hurry…! I’m losing it… aah, ah… Onii-chan… nn… Onii-chan…!”

Pinning her hands, I thrust into her, still clothed. Realizing this was her first-year uniform reignited the taboo thrill of fucking my sister, crawling up from my balls. Her T-shirt, stretched tight even lying down, showed how erotically she’d grown. I needed to fill her with my semen.

“Miyu… here it comes… fuu, u… aah…!!”

Biyu, byugu, byururu—thrusting hard, I unloaded deep inside her. Already out of it, she just basked in the warmth in her womb, but without the shame play, this creampie could’ve addicted her to the sensation.

“U… uhh…”

To avoid staining her shorts—extra fuel for her solo sessions—I pulled them aside. There lay my sister, half-undressed in her uniform, semen dripping from her slit. Resisting a second round, I snapped a photo with her phone and left it at that.

I don’t know what she did with the picture—she didn’t mention it after waking. Maybe she hadn’t noticed. That night, the uniform didn’t hit the washer, so, respecting her hobby, I let exhaustion take me to an early sleep.