How Using My Oppai Skill to Enlarge My Sister’s Breasts Led to Building a Harem - Chapter 21 Part 2

Chapter 21 Part 2

Since it’s Christmas, let’s have a festive story.

Don’t worry about the timeline.

It’s Christmas Eve.

Even so, I’m diligently attending classes. My immediate goal is to pass the entrance exam for the country’s toughest university.

But of course, outside of class, I’m having fun with the girls—no surprise there. Right now, Hidaka Riria, sitting in front of me, is poking my arm.

“Hey, hey, let’s have a Christmas party!”

“What’s that?”

“Christmas party! My place is empty tonight, so Yukino-chi can come too.”

“Me too?”

“Of course♪ We’re besties, right?”

“Thanks, Riria-chan!”

I’m not sure about Yukino being besties with Riria, but she looks happy, so I keep quiet.

Riria’s the type to flirt or harass whenever she gets the chance. She’s not a bad person, just… unconventional.

“A party, huh…”

Come to think of it, I haven’t done something like this since I was a kid. Last year, I was a loner with no friends, so no party invites came my way.

“Just the three of us?”

“Invite whoever you want.”

“Hmm.”

Ai’s at a friend’s house for a girls’ night. Satsuki-chan’s working hard today for our date tomorrow. Ayaka-san and Miyabi-san are adults, so inviting them to a kids’ party feels off. But one person comes to mind.

“Can Nao come?”

“Wanna invite her?”

“Totally cool.”

“Then I’ll ask her later.”

With Riria’s approval, Nao’s invited.

…

“Heya, Yuki-nee! Onii-chan and Riria-chan too!”

“Nao-chan’s full of energy.”

Despite the late December chill, Nao’s in a miniskirt uniform with just a jacket.

“I’m tough♪”

“Honestly, Nao…” Yukino frets.

Is that just family worry? I’m honestly thrilled to see Nao’s bare legs.

“You cold, Onii-chan?”

“Yeah, it’s winter. Kinda chilly.”

“Then I’ll do this♪”

Nao clings to my right arm.

(So soft… she’s really filled out.)

When we first met, Nao’s breasts were much smaller. Now they’re lush enough to call huge.

“Then I’ll… Ei!” Yukino grabs my left arm.

Her breasts, just as bountiful as her sister’s, envelop my arm. Both sisters have ideal breasts, wrapping my arms even through clothes. Sandwiched by beauties, I struggle to keep my composure.

“You forgetting me?” Riria pouts.

“What, you want the front?”

“…Idiot.”

I can’t walk with a girl hanging on my front—that’d make me look like a pervert in public.

“I’ll switch, Riria-chan,” Yukino offers, letting go.

“I don’t care about arms…” Riria sulks.

“I wanna cling to you too, Riria.”

“…Fine, I’ll let you.”

She grabs my arm.

(Palm-sized. Less volume than the sisters, but they suit Riria’s free spirit perfectly.)

Riria’s fashion-conscious, avoiding overly large breasts that might bulge her clothes.

“You’re totally into this,” she teases.

“Ugh!”

Suddenly, she grabs my cock. The surprise makes my half-erect member twitch.

“Stop, I can’t walk…”

“Nishi, pervert♡”

“I wanna rub too!” Nao chimes.

“Seriously, stop…”

Until we reach Riria’s place, I’m their toy, girls taking turns clinging to me.

…

We stop at a supermarket for food, then head to Riria’s.

(Gotta spend sometimes. I don’t usually, so I should play it cool.)

The Saionji family handles my grooming—clothes, everything. Since starting strength training, I’ve cut out snacking. Gaming’s less appealing than hanging with girls. So, I pay for occasions like this.

“We’re here.”

Riria’s home is a modest, older apartment.

“Mom’s at her new boyfriend’s, so she won’t be back for a while.”

A single-parent home, and her mom seems as free-spirited as Riria.

“My brother’s in college, living alone, so it’s chill.”

She laughs, but her smile seems lonely. I get why she wanted to throw this party.

“Pardon the intrusion.”

It’s a modest 3LDK, not too spacious, contrasting her flashy appearance.

“Surprised it’s so small?”

“Nah, it’s fine.”

It’s not shocking. Without knowing her bold style, no one would bat an eye. Riria’s known for heavy makeup and piercings—navel, nipples. Living in a normal apartment feels like a gap.

“My place runs on alimony for me and my brother.”

Divorce, maybe? I hesitate to pry.

“I barely got an allowance, so I started selling myself in middle school.”

I figured as much.

“But I stopped recently… Thanks for covering earlier.”

“Don’t sweat it. Guys pay on occasions like this, right?”

Depends on the person, and I hate being used, but I want to show off for someone I like.

“That’s Onii-chan♪ Such a provider,” Nao teases.

“Haha, sometimes, right?”

If it makes them happy, it’s worth it. I want to help Riria if she’s struggling financially.

(I’d rather she didn’t sell herself…)

I’m not judging, but the thought of her with other guys bugs me.

“Gonna rely on you lots, nishi♪”

“Hey, hold up.”

Her smile’s back—not the lonely one, but genuine.

…

“Yay! Drinking?!”

“Woo!”

“Gulp, gulp~”

Hours later, three tipsy girls are in full swing. For the record, I didn’t buy alcohol.

But I had no control over what was already here.

“Yukino, stop already!”

“You’re so serious~”

I didn’t notice at first. The orange juice they drank had stuff minors shouldn’t touch.

“Spike it gradually so they don’t notice—that’s how you get a girl,” Riria mimics her brother. What a guy…

Too late. Yukino and Nao are plastered.

(My size helped me notice before getting drunk…)

“Drink more~” Yukino slurs.

“Haha, Yuki-nee’s hilarious!” Nao laughs.

How’d this happen? It started with school talk and girly trends, all pink and fluffy.

“Alright, time for the main event!” Riria declares, dragging Yukino and Nao to another room.

(Main event? Why am I left out?)

I wait, wondering, until—

“Ta-da! How’s this?”

Three adorable Santa girls appear.

Wearing risqué red-and-white outfits, Riria, Yukino, and Nao strike poses.

(Whoa… their underwear’s fully visible under those miniskirts…)

The fabric’s tight, soft breast flesh spilling out. Yukino tugs her skirt shyly, while Nao spins, flashing her panties.

“Wow… you’re all super cute. Like idols.”

Honestly, it’s irresistible. The outfits are probably standard-sized, but they can’t contain the Tsukihara sisters’ curves.

“How’s mine?” Riria asks.

“Super cute, Riria. Shows off your great figure.”

“Fufun♪”

She’s not outdone. Her natural beauty and well-maintained body could make her a model.

(Damn, I’m getting hard…)

Maybe it’s the drinks, but their energy’s high, faces flushed, exuding a sexier vibe than usual.

“No touching, mister~” Riria teases, grabbing my hand as it instinctively reaches out.

“Ugh…”

Three gorgeous girls, and I’m on hold? My pants are about to burst, and it’s unfair to tempt me like this.

“You gotta pay first♪”

“What, money?”

How much? I don’t carry much cash, and I spent a lot today—maybe 3,000 yen left. For cosplay sex with these three, that’s a steal, but probably not enough.

“No way~”

“???”

“Your payment… is your C-O-C-K♪”

Cock payment? I didn’t know it had that value, but it’s basically free. My answer’s clear.

“Please, go ahead.”

“Right this way~”

Riria pulls me to her room.

…

“Take it off!”

“Yeah, yeah.”

No need to tell me. My cock’s raging, trapped in tight pants. I fumble with my belt, yanking my pants down.

“Wow♡ Bigger than usual♡” Riria gasps.

“Well, with outfits like that…”

School quickies are rushed, less satisfying. I rarely get to savor Riria’s erotic form like this.

“You’ve been training, huh? Looking good.”

“Yeah, got some muscle.”

“You’re taller too.”

“Almost 180 cm.”

Thanks to Miyabi-san’s pro-athlete protein drinks, perfect for my growing body.

“Your cock’s huge too.”

“Is that a compliment?”

Better than being called small, but it’s odd in context.

“You’re getting better at studies too, right?”

“Still a ways to go.”

I barely cracked the top five in the last test. For my goal, topping a regular school is the minimum.

“No guy’s better than you.”

“Nah, there’s gotta be.”

Soccer team heartthrobs, band playboys…

“Your face is my type.”

“Didn’t know that.”

I’m not confident about my looks. Being Ai’s brother doesn’t make me a heartthrob.

“It’s like you’re seducing me.”

“…I am.”

“Huh?”

Exactly that.

“I’m pretty serious.”

“Why me?”

“Dunno. Just fell for you…”

Her confession’s more earnest than I expected. I’d be lying if I said I wasn’t happy. She’s wild but kind, and super cute. I’m… really into gals like her.

“But I’m…”

“I know. I saw at your place.”

Siscon, incest, cheating with a married woman despite having a girlfriend. Wait, am I kind of a bad guy?

“I don’t care about the details.”

“Figures.”

“I don’t need to be number one or marry you. Screw around with other girls all you want.”

“Uh, yeah.”

“So…”

Her face is red. Usually so elusive, now she’s vulnerable.

“Like me too…”

She looks up at me with doe eyes.

(She’s so cute. Whatever her past, she’s insanely attractive now.)

What guy wouldn’t fall for this? None.

“I already like you.”

“Really?”

“Yeah, you’re crazy cute, Riria.”

I can’t hold back. I want to make this adorable girl mine. My erect cock, leaking precum, strains upward, eager to take her.

“Not just talk, huh? Here♪”

“Ugh…!”

She grabs my cock roughly, stroking it up and down. Her slender, pale fingers wrap around my shaft.

“Gua… aa…”

“Here, shiko shiko shiko~♡”

How’s a girl better at this than me? Her handjob feels incredible.

“Riria…”

“Mm~♡ Wanna do it? Want a blowjob?”

She sticks out her tongue, mimicking a blowjob, teasing me.

(Damn, I’m so horny I’m losing it…)

My lust’s clouding my mind, and it’s her fault for egging me on.

Yeah, her fault.

No way I’m letting a blowjob finish me—my lower half agrees.

“Then I’ll do you, Riria.”

Time to have some serious fun.

…

“How’s it, Riria? Fun screwing me?”

Riria gets off on rough sex, so my words turn crude.

“Ngo♡ So intense…♡”

Guchu♡ Guchu♡ Guchu♡ Guchu♡

I pin her from behind, ramming my cock into her ass.

“Aa♡ My ass, it’s breaking♡ Hii, nooo…”

Each thrust stirs her insides, making her shake her head. It might look like I’m forcing her, but it’s not. It’s an act.

Her rectum’s been squeezing my cock since I entered, milking me multiple times. My strange power lets me know she’s feeling good.

“Liar! How many guys have you taken in this hole?”

“Auu♡ Never a cock this huge…♡”

She might say that to everyone. I don’t care about her past, but if we’re getting closer, I want to claim her.

“Then practice fitting my size!!”

Nubo♡ Dochun♡ Nubo♡ Dochun♡

I lift her body, slamming it onto my rock-hard cock like a skewer.

“Ogo♡ Aga♡ Hii, hiii♡”

I pound her ass, using my power to teach her pleasure.

Dochun♡ Dochun♡ Dochun♡

“Hau♡ What’s that… so good♡ Ngii♡”

Her intestines have sensitive spots, deep where my cock barely reaches.

“Here? You love it when I scrape here?”

“Aga♡ No…♡ I’ll really lose it!”

Her moans aren’t acted anymore. She’s losing control.

“I’m making you mine, Riria!”

Doju♡ Grigrigri♡ Dochun♡ Dochun♡ Dochun♡

“Kaha♡ Impossible, too much…♡ My head’s breaking♡”

“Lose it. Forget everything and feel good.”

“Oh♡ That’s the best♡ Aa, I’m cumming… cumming cumming cumming!”

Pleasure swells in her body.

“I’m cumming too…!”

I’m at my limit. Her body’s a masterpiece built for sex. Despite my bravado, I’ve already shot multiple times.

Doju♡ Doju♡ Doju♡ Doju♡ — Dojun♡♡♡

“A…♡ Cumming, guuu~~~♡♡♡”

Her body trembles, vibrations traveling from her anus to her intestinal walls, stroking my cock.

“Gu, oooHH!!”

Her ass’s vibrations caress my cock. My glans swells, pressing her sensitive spots.

Dopyu♡ Byuru byururu♡

“Ngo♡ So good… hii… auu♡”

“God… your ass-pussy’s amazing, Riria.”

I cum hard in her intestines, in heaven. And I understand her.

Her open-mindedness about sex. A body made for it. Craving stimulation deep in her core.

If there’s talent for sex, she’s a genius. She could become a global porn star.

(But I want her.)

I’ll become the man to satisfy her. A woman I admire taught me: to get what you want, improve yourself.

With my lofty goals, this is just one more.

“Riria, we’re gonna keep screwing!”

“An, hold on…♡”

I’ll make her happy. With that resolve, I swell my cock.

…

“How’s it feel?”

“Aa♡ My pussy’s loving it♡”

My cock’s loving it too, pumping semen into her pussy against my will.

“I’ll never get tired of this.”

“Best compliment ever♡”

“I love you, Riria.”

I look into her eyes.

“I love you too♡”

Chu

A rare kiss with Riria, tasting like happiness.

※

What happened next?

“Riria-chan, you’re taking too long!”

“We’re waiting to screw Onii-chan too!”

As Riria goes down, Yukino and Nao barge in, turning it chaotic.

“Alright, one at a time.”

“Yes.” “Yup.”

…

“Aan…♡ Your cock’s so good♡”

“You’re so cute, Yukino.”

Dobyu♡

I ravage Santa-girl Yukino—

“Onii-hyan♡ Love you, love you♡”

“So cute, Nao…”

Dobyu♡

And reindeer-girl Nao.

“Haa… Satisfied… No more semen left.”

Three in a row, I went all out. I’m done, I think.

But—“What’re you saying? My turn!”—Riria’s back.

“You won’t say you can’t with me, right♪”

I vowed to make her happy. No running.

“I’ll make you roll your eyes again, Riria.”

“Looking forward to it, nishi♪”

I’ve long hit my limit, but I decided to be the best at sex too. That’s my way of love.

※

“Oh, it’s midnight?”

While I took turns with the three beauties, it got late. Yukino noticed first.

“Oh, true.”

“Didn’t notice.”

December 25th. Japan’s holiest night for sex. We’re fully indulging in this Christmas sex event.

And there’s a traditional greeting for this sacred night. My beloved girls, after our wild romp, look at me and say:

“““Merry Christmas ♥♥♥”””

…

As an aside, I faced my date with Satsuki-chan the next day, completely sleep-deprived.