How Using My Oppai Skill to Enlarge My Sister’s Breasts Led to Building a Harem - Chapter 1

Chapter 1

T/N: Another incest yeah, I missed My Little Sister is the Best Fap Material LMFAO. And yes, this will be a proper harem in a modern setting. There are some ero illust for this novel, AI illust actually, made by the author, but for various reasons I won’t be uploading them here. You may check out the raw yourself or wait until I finish setting up my Ko-Fi page, I’ll upload pdfs there and epubs for free.

My little sister is in her third year of middle school. Her name is Ai, a boyish beauty with short, brownish-black hair. But there’s one thing that sets her apart from other girls—a pair of massive, unmistakable mounds on her chest. Her oppai.

“Onii, playing games again? Why don’t you exercise for once?”

“Maybe later, Ai…”

I’m just a high school slacker, part of the go-home club, while my sister’s a track star. To think she’s the ace of the sprint team with a 96cm bust—those enormous oppai—it’s nothing short of incredible.

※

But there’s a secret behind how Ai got such magnificent oppai.

“I shall grant you one special skill.”

I can still faintly recall those words, heard when I was barely born.

And that special skill? It’s oppai.

As a kid, I had no idea what that meant. But as I grew older, I started to understand—I could make someone feel good by rubbing their oppai.

※

“Ai! Onii-chan’s gonna give you a massage!”

“A massage? What’s that?”

“Like this!”

“Kyannn♡ It tickles!”

I must’ve been around ten back then. Under the guise of “massages,” I kneaded my sister’s chest every day. And the result? Ai’s breasts grew into what they are now.

You might think it’s a coincidence. But our family? Flat as boards, every last one of them. Sad, pancake-level. Yet Ai alone swings around these colossal fruits.

Can you really call that a coincidence?

※

Gokku, gokku… puhaa! “Nothing beats coffee milk after a bath, right?”

Fresh from the shower, Ai struts around the living room in a tank top and hot pants, her casual outfit clinging to her. Her damp hair and tanned, wheat-colored skin gleam with health, dazzlingly vibrant.

But my eyes? They’re drawn elsewhere—

“Want some, Onii?”

“Yeah.”

“Here ya go.”

Ai saunters over to the sofa where I’m sprawled, setting a glass on the table. In that moment, her tank top shifts, and from the side—purun purun♡—I catch a glimpse of those massive, jiggling mounds of oppai. The floral scent of shampoo and body soap mixes with that sweet, distinctly girly aroma. A middle schooler with that kind of bust? I can’t help but sneak glances, my heart pounding despite her being my sister.

“Thanks.”

“No biggie.”

I’m not just saying this because she’s family—Ai’s got a great personality too. Kind even to a creep like me, who gets flustered over his own sister. She’s a popular honor student at school, the total package.

(How did we end up so different…?)

A gaming-obsessed loner like me couldn’t be further from her orbit. I’ve been living with this inferiority complex, trudging through a mundane life—or so I thought.

※

“Onii, you really like oppai, don’t you?”

“Buhooo!! W-What’re you saying all of a sudden?!”

Coffee milk sprays from my mouth and nose, and my heart nearly leaps out of my chest.

“I mean, you’re always staring at my chest.”

“N-No, I’m not…”

“Oh, I can tell.”

“…”

Okay, fine, I love oppai. I’m so obsessed that I can’t even relate to butt guys. It’s oppai or nothing.

“…Sorry.”

“I’m not asking for an apology. It’s just—”

“Hm?”

“—you used to rub them a lot when we were kids, right?”

“T-That was…”

She remembers. I started rubbing her chest around age ten. Within six months, she was a C-cup. A year later, F-cup. I realized it was getting out of hand and stopped.

“I’m not mad or anything! But, like, maybe that’s why they got so big?”

“Uh… maybe, yeah.”

“Right? Okaa-san’s flat as a board.”

“Don’t ever say that to her face. She’ll kill you.”

Seriously, she’d murder us.

“So, um, I have this friend who’s super worried about her small chest…”

“Hm? Oh…”

“She was wondering if rubbing them would make them bigger.”

What do I even say? If I admit I can make oppai grow, I’d sound like some kind of 変態 (hentai).

“Depends on how you rub, I guess…”

“That’s what I thought! She said rubbing them herself did nothing.”

“Well, yeah, that tracks.”

“So you can make them bigger…”

I get the urge to retort, “You believe that?!” But Ai’s got firsthand proof—my massages worked on her.

“Okay… how about you rub mine again? Just once?”

“Wha?!”

I stare at Ai’s face, shocked. She’s serious, though her cheeks are a little flushed.

“You for real?”

“I mean, I need to confirm it myself…”

She’s not kidding. She leans closer, her trembling oppai—furufuru♡—within arm’s reach. I hesitate for a split second, but those dreamy mounds, packed with every man’s fantasy, overpower me.

Momi♡ Guniguni♡

“Haaaan♡ Onii, that’s intense♡”

“Fuu, fuu… You’re pretty intense yourself, Ai… Here, here!”

Guni♡ Gunyugunyu♡ Guniii♡

“Aahhh♡ So good, so good♡ My oppai are tingling inside♡”

“Gotta stimulate deep down like this.”

My fingers move intricately, hitting her oppai from every angle. Those sensitive nerves—I can feel them under my touch.

Gui♡ Munyuuu♡ Guniguniii♡

“Aaah♡ No… I’m gonna… I’m gonna come~~~♡ Nnnuuu♡♡♡”

Ai twitches, her body shuddering. Her breath hits my face, filling my lungs with that sweet girly scent. I get carried away, craving more of her oppai.

“Clothes are in the way. Off they go!”

“H-Huh? Kyaaaa!”

I lift Ai’s tank top, revealing her bare breasts. Slightly brownish areolas and nipples greet my eyes.

“Onii! I didn’t say you could look at my nipples!!”

“…Sorry. But, weren’t they pinker before?”

“~~~!? That’s why I didn’t want you to see!”

Nipples darken because of melanin. Friction, UV rays—Ai’s track practice, with all that rubbing and sun exposure, probably caused it. Once pigmentation sets in, it’s tough to reverse. But…

“I might be able to fix it.”

“Really?”

“Yeah… I can.”

“…Okay, maybe just a little.”

Blushing, Ai lifts her tank top herself. Those massive mounds and darkening nipples reappear before me.

“Here goes.”

Juuu♡ Chuuuupa♡

“Ahiinnn♡ Wait… auuu♡”

Jupa♡ Jupa♡ Rerorero♡ Juzuzuzu

“Onii… this is… too much♡”

I grip her oppai from the base, milking them upward, and latch onto her nipple, sucking as if drawing out the pigment, licking it all over.

“Don’t talk while sucking♡”

Jururu♡ Juppa♡ Rerorerorero♡ Juppappa♡

“Aaah♡ I’m coming♡ I’m coming again~~~♡”

Ai hits her second climax, trembling. I’m filled with satisfaction, having devoured her oppai to my heart’s content.

“Haa… haa… Onii, you went too far…”

“Can’t help it.”

I’m a guy, after all.

“No way this makes nipples prettier…”

“Nah, look.”

“What?!”

The nipple I sucked is noticeably lighter. Not perfect, but compared to the other, the difference is clear.

“No way… it actually worked?”

“Your Onii-chan’s good at this stuff.”

I puff up with pride. It’s not like I have many talents, so let me have this.

“Wow… it’s amazing. I’ve always been self-conscious about the color…”

“You’re a girl. Wanting to be pretty is normal.”

“Yeah… thanks, Onii.”

Ai shyly thanks me, her face so cute it makes my heart race.

“But we’re not done, right? You’ve got two nipples.”

“Wha?! Wait… I need a break!”

I just want to see more of her adorable face.

“Here we go, Ai.”

“Nooo♡ Ahiinnn♡”

And so, Ai’s oppai treatments became a regular thing.

※

A few days later, after finishing with Ai’s nipples, she brought up her friend again.

“So, about that girl who’s worried about her small chest…”

“Oh, yeah, you mentioned that.”

It was real?

“She wants you to rub hers.”

“What?! For real?!”

“So, I’m bringing her over tomorrow, okay?”

And that’s how I ended up rubbing Ai’s friend’s chest.

“Yo, I’m Sasahara Satsuki, Ai’s friend♪”

“Uh, nice to meet you.”

The girl Ai brought is cute, with a slight gyaru vibe, all bubbly and light. But her chest? Doesn’t seem that small.

“My boobs are so tiny…”

“Are they?”

“Wanna see?”

“Uh, sure.”

I feel weird checking out her friend’s chest with Ai watching, but I’m still a guy who wants to see a cute girl’s oppai.

“Here!”

No shame at all, Satsuki strips off her uniform top, revealing a mature black bra.

(B-cup, maybe? A bit small, but not nothing.)

“They’re pretty.”

“Haha, thanks! But I want them bigger.”

“I see.”

“Can you do it?”

Of course I can.

“Yeah.”

“Cool, I’ll take off my bra.”

“…!”

Satsuki unhooks her bra, revealing perfectly shaped oppai. Small nipples, slightly darkened.

(Does she play with them during…?)

Just a hunch.

“Alright, let’s get started.”

“Go for it♪”

She’s totally cool with it. Rubbing a girl’s oppai who isn’t my sister is a first. I’m a bit nervous.

“Nnnu♪ Oh my gosh, you’re too good at this!”

“Am I?”

I pinch the gentle slopes of her modest chest. Punipuni♡—they’re definitely a girl’s oppai, soft and elastic. I massage them gently with my palms. Compared to kid-Ai, these are big enough to work with easily.

Finyu♡ Kyuuu♡ Munyu♡

I vary the pressure, stimulating her nerves just right.

“Faaah♡ My oppai feel hot inside…♡”

“That means it’s working.”

I knead her nipples for stronger stimulation.

Kuri♡ Korikori♡ Kyu♡

“Aaah♡ That’s so good♡”

A gorgeous girl I just met is moaning under my touch. I’m thrilled, keeping up the pace, rubbing her chest and teasing her nipples.

“Fuuaah♡ I’m gonna… I’m coming~~~♡ Aahhh♡♡♡”

Satsuki arches back, twitching. Then she slumps, exhausted.

“Let’s call it a day.”

“Haa… haa… Will they get bigger?”

“They will. Do this daily, and you’ll see results soon.”

“Cool. I’ll come back tomorrow.”

For three days, I massaged her oppai the same way.

“You’re amazing! They grew a whole centimeter!”

Puberty? Her potential? Either way, her oppai are growing faster than I expected.

“So, Ai said you can make them prettier too?”

“Yeah.”

“Can you do it?”

“Sure.”

I squeeze her oppai—munyuri♡—and kiss the pointed tips.

Jururu♡ Juppa♡ Chuppappa♡

“Nnhiii♡ They’re being sucked up♡”

“Sucking draws out the pigment.”

I keep going, savoring her oppai.

(A girl this close in age… and so pretty…)

Jupa♡ Jupa♡ Rerorero♡ Juzuzuzu♡

Her sweet moans and youthful pheromones make my groin stir, mukumuku.

“Your thing… it’s getting big.”

I try to ignore my tight pants, focusing on licking and rolling her nipples.

“Nnn♡ I’m gonna… I’m coming~~~♡ Aahhh♡♡♡”

Satsuki trembles, a bigger climax than before. A thick, girly scent—muwari♡—wafts from her groin. Inhaling it, my cock gets harder than ever.

(I’m so turned on… I could go multiple rounds right now.)

With Ai here, I can’t, but if we were alone… who knows?

“All done, Satsuki-chan.”

“Haa… haa… Thanks.”

I play it cool, promising another session tomorrow.

(Daily oppai sucking? This is the life.)

I kept it up, day after day.

A month later, I’m waiting for Satsuki-chan, giddy on the living room sofa.

(She’s up to a D-cup now. Her nipples are pinker too. She’s a diamond in the rough.)

The door opens, and Ai comes in.

“Um, Onii…”

“What’s up?”

“Well… Satsuki-chan says she’s done…”

“What? Done with what?”

I don’t get it. She was so desperate to grow her oppai…

“She got a boyfriend. Some guy from the soccer club.”

“What?”

“He’s into big boobs, so she asked you to help.”

“…”

So I was just a masseuse, not a love interest. Couldn’t she at least say thanks? Got a guy and just ditches me?

“I see… Got it.”

“Sorry, Onii…”

“Why’re you apologizing?”

“Because…”

“Can I be alone for a bit?”

I’m crushed. Was it love? I don’t know. But I took pride in “raising” her oppai, and I thought she felt it too. Guess I was wrong.

“Damn it!”

I slam my desk—don—in my room. To shake off the frustration, I start my daily routine, but it’s no good. I finish without much satisfaction.

“Sorry, Onii…”

Sighing heavily, I crawl into bed to forget it all.

(No more girls’ oppai starting tomorrow…)

The thought sinks my mood to rock bottom.

※

“Onii, you’ve been down lately.”

“Have I?”

“Yeah.”

It’s been a few days, and I’m definitely off. The idea of Satsuki-chan using her oppai—the ones I grew—to seduce some guy, maybe doing even wilder things, pisses me off.

“If it’s okay with you…”

“Hm?”

“Wanna rub my oppai?”

“What?”

“If you don’t want to, it’s fine.”

“Is that okay?”

“It’s kinda my fault…”

I stare at Ai’s face. She blushes and looks down.

(She’s so cute. If she wasn’t my sister, she’d be way out of my league.)

“It’s not your fault.”

“So… you don’t want my oppai?”

“No, I want them.”

Ai giggles, her face lighting up. My heart leaps—dokun—and my blood pressure spikes.

“Your face is red, Onii.”

“So is yours.”

We lock eyes, faces inching closer. Ai’s pale pink lips tremble, brushing against mine.

“Nn… chu♡ Nnn♡ Churu♡”

“Churu, Ai… you’re so cute.”

“Haan♡ Onii♡”

Our clumsy kisses deepen, tongues tangling, swapping sweet saliva. With every taste, I start to see Ai not as my sister, but as a woman I love.

“I’m taking this off.”

“Ah, yeah. Hya♡”

I pull up Ai’s tank top. Her heavy breasts lift with the fabric, then drop—bain♡—bouncing with momentum.

“Perfect oppai…”

Size, shape, softness, and those pinkened nipples. These are the ultimate oppai, the pinnacle of humanity’s dreams.

“Because you rubbed and sucked them…”

“Gonna love them even more.”

“Hyan♡”

I knead her oppai with practiced hands, squeezing from base to tip—guniguni♡—toward her nipples.

“Onii… it’s all tingly…”

“Leave it to me.”

I tease her nipples, now stiff with anticipation, pinching them—kuri♡.

“Auuu♡ Haaa♡”

Ai climaxes just from that, trembling, her face melting with pleasure.

“Ai… chupa, nn.”

“Nnu, Onii♡ Jururu… chupa♡ Au♡”

I toy with her nipples while kissing her deeply, our tongues entwining. Ai responds, matching my rhythm.

“Reru♡ Juzuzu♡ Afu… Onii♡ Hyau♡”

The girl before me, flushed and embracing pleasure, ignites a deep, primal urge in my hips.

“Fuu, fuu… Ai.”

“Onii, you’re getting big, aren’t you?”

“Whoa!”

Ai, sensing my excitement, strokes my groin—shururi.

“Nfufu♪ You’re so excited.”

“Because you’re too cute.”

“Then I’ll make you feel good too. Here!”

“What?”

Ai yanks down my lounge pants and underwear. My cock springs out, grazing her face, standing tall.

“W-Wow?! It’s so… perky♡”

“Ai, don’t stare so close…”

She’s inches away, her breath teasing me as she studies it.

“We used to look at it all the time when we were kids♪”

“We’re not kids anymore…”

“Right… it’s so big and hard now…”

Her slender fingers grip my cock. The first time a girl’s touched it, I let out a pathetic “Ugh.”

“It feels good just being held♪”

“Ah… ugh…”

“Like… up and down, right?”

Using half-remembered knowledge, Ai starts stroking—shuko shuko♡. Her soft hands, clumsy but eager, rub my shaft.

“How’s that, Onii? Am I doing it right?”

“Haa… haa… Feels good, Ai.”

It’s not skilled, but that rawness is what makes it so intense. But if this keeps up, I’ll finish in her hand…

“Ai… can you use your oppai?”

“With my oppai? Like, sandwich it?”

“Yeah…”

“Okay♪ Here!”

Munyuuu♡ Her soft breasts envelop my cock, squeezing with incredible pressure.

“Uaaah! Amazing, Ai. It’s like my cock’s melting!”

“Your c-cock… it’s so hot♡”

Her 96cm bust toys with my cock, completely at her mercy.

“Here, here♪ You love my oppai best, right?”

Ai shakes her breasts side to side, up and down, rubbing them together alternately.

(This is too good… my hips are gonna give out…)

“Your cock’s getting hotter♡”

Ai’s lost in stroking me with her oppai. I can’t let her make me finish alone—that’d be embarrassing.

“Here, here♡ Your cock’s twitching♡ Come on, come♡… Fugiii♡”

I pinch her nipples hard.

“Nooo♡ Don’t tease my nipples like that♡ Hauuu♡”

I sink my thumbs into her nipples, piercing her oppai.

“Auu…♡ That’s… nooo♡”

“Shake your oppai more.”

“U… okay. Hiu♡ Au… Ihiii♡”

Each shake buries my fingers deeper, hitting her pleasure points. We’re both hooked on the sensation, moving faster.

“Good, Ai! Keep stroking with your oppai!”

“Nnhiii♡ My oppai are going crazy♡”

“I’ve got you. Feel as good as you want!”

“Onii, Onii♡”

“Kuh… ugh!”

Before Ai climaxes, my cock hits its limit. The pent-up desire surges, bursting out as thick, white liquid—byuku byuku♡—coating her oppai.

“It’s coming out♡ So much♡”

“Haaa… Still coming…”

I thrust my hips, reveling in the sticky mess of my own release. My first sexual experience, with the oppai I love most.

(Haa… that was amazing. I could die happy now.)

Sated, I pull my cock from her oppai—zururi—and wipe it clean with tissues.

“That was incredible, Ai.”

But Ai isn’t done.

“That’s not fair, Onii… fuu fuu♡”

She strips off her shorts, revealing a smooth, hairless slit. A glistening thread of liquid drips from it, proof of her arousal.

“Hey, you can keep going, right?”

Kuchukuchu♡ She’s touching herself, impatient.

I swallow hard. The girl who was just drenched in my cum is now tempting me with her pussy. But she’s my sister.

(Can I cross this line…?)

Once I do, there’s no going back.

“I love you, Onii.”

“I love you too, Ai.”

“Then take my first time.”

Ai stops touching herself, spreading her slit with two fingers—kupaa♡. It’s a vivid pink, untouched, pure. The sight is too intoxicating, and my male instincts take over.

“Ai… I can’t hold back anymore.”

“Yeah♡”

I press my newly hardened cock against her pussy, pushing the tip in. It slips in—nyuru—then catches.

“Come on, Onii♡”

“Here I go, Ai.”

I feel something tear as my cock slides into her.

“Kuh… aaah…”

“You okay, Ai? I’ll be gentle.”

“U… thanks.”

I stroke her head, waiting for the pain to subside. We hold each other, my cock halfway inside her tight pussy.

“I’m okay now, Onii.”

“Really? Tell me if it hurts.”

“I will.”

I move slowly, opening her tight pussy. Finally, I hit the deepest part—kotsun—and we’re fully connected.

“It’s all in.”

“Yeah, we’re…”

She trails off, but I understand. I’ve made up my mind.

“Move, Onii.”

“Got it.”

To ease her pain, I massage her oppai, hitting the spots I know she loves—guniguni♡.

“Aun♡ Feels so good, Onii♡”

“I’ll pamper you plenty.”

Her pussy grows slick, making my cock slide easier—pachun♡ pachun♡.

“Haan♡ Onii’s going in and out♡”

“Feels so good, Ai.”

I caress her oppai while thrusting, stimulating her insides. Ai starts moving her hips too, the pain gone from her expression.

“Nnhhaa♡ So good♡ This is sex♡”

“Your insides are too good, Ai.”

“Nfufu♪ You can go harder♡”

Paan♡ Paan♡ Pachun♡

“Naaah♡ More… thrust harder♡”

Paan♡ Pachun♡ Pachun♡

I give in to her demands, her athletic body matching my rhythm, squeezing my cock—gyuuu♡.

“Kuh, so tight.”

“More, Onii, more♡”

Her melted expression begs for my cock. Our bodies, so perfectly matched, lose all reason as we grind together.

Dochun♡ Dochun♡

“Nohoo♡ Onii♡ That’s crazy♡ So good♡♡♡”

I pound into her pussy from above, our bodies locked together, the perfect match.

“Ai, Ai, I can’t stop!”

“Ohh♡ Amazing♡ My head’s going dumb♡♡♡”

Dochun♡ Dochun♡

“Fuhn! Fuhn! How’s that, Ai?”

“Ohhh♡ Onii♡ I’m… I’m gonna fly♡”

“Go for it! Come as much as you want!”

Dochun♡ Dochun♡ Dozun♡

“Ohhh♡ I’m coming♡ Coming coming coming~~~♡♡♡”

“Uuu… aaah!”

As Ai climaxes, I pull out, shooting my load across her toned stomach and chest—byuru byururu♡.

“You pulled out…”

“Can’t risk a baby.”

“Yeah… true.”

I wanted to finish inside, but that’s a line we can’t cross.

“But… someday, let’s make one.”

“Yeah.”

We hold each other naked, kissing repeatedly.

※

After our first time, we got addicted to sex.

“Ai, Otou-san and Okaa-san are both out today.”

“Yay! We can have tons of sex now♡”

“Yeah, Ai!”

“Nn, chu♡ Nnn♡ Jururu♡”

We strip, put on a condom, and I slide into her pussy.

Paan♡ Panpan♡ Panpanpan♡

“Nnhiii♡ Onii♡ Feels so good♡”

“You’re making me feel good too, Ai.”

We sneak sex whenever our parents aren’t around. With them working late, our pent-up lust drives us to ravish each other daily.

“Okaa-san might come back soon♡”

“We’ll just head to my room.”

“Aaahn♡ So intense, Onii♡♡♡”

During summer break, with the house to ourselves, we did it everywhere—living room, kitchen, bathroom, toilet.

※

“Onii, we’re out of condoms.”

“Condoms aren’t cheap, huh?”

Even buying a hundred online wasn’t enough for how much we went at it.

“Wanna use my oppai?”

“That’s great, but turn around for a sec.”

“Okay.”

“Let’s try here.”

I tease her butt hole with my finger.

“Hya?!”

“I’ve got lube. Let’s try it.”

“For real? Isn’t it… dirty?”

“My world’s cutest sister doesn’t have a single dirty spot.”

I say it with confidence.

“Uuu… Okay, let’s try.”

Ai blushes, staring at me. Her cuteness never fades, no matter how many times we do this.

“But… you’re the world’s best too♡”

Those words make me unstoppable.

“Ai! We’ll be together forever!”

“Yeah♪ Forever and ever♡”

We sank deeper into pleasure, craving each other. But I have no regrets.

Because we’re the most in love in the whole world.