Little Sister's Assault

"Gahhh!!!"

I clutched my head and rolled around on the bed. I couldn't properly remember exactly what I'd been thinking last night or what depraved thing I'd done. But one thing was crystal clear: a sinister certainty that I'd done something lewd... no, downright filthy with this body—something I absolutely shouldn't have even imagined.

"Why the hell? Why would I do that?!"

The bedsheets and my pajamas were pristine, as if nothing had happened—like they'd just come out of the dryer. If someone saw me now, they'd probably think I'd just had a nightmare or something. Everything looked that peaceful.

I'm losing it. I must really be going crazy, Irina.

Irina. No, Irina...... Sighing deeply, I repeated the name that had become mine. I had no idea what had happened overnight, but the unpleasant sensation lingered vividly, shaking my mind to its core.

"Is that the only greeting you know?"

The same old pattern every time was starting to get on my nerves, so I snapped at it. Honestly, I was bracing myself for whatever bullshit it would spew next, but surprisingly, it didn't respond.

...What the hell?

Did it actually get hurt or something? A crying emoticon just flickered weakly on the blue window, saying nothing else.

But whatever. Did I care? Compared to all the crap it'd done to me, this was nothing—it didn't deserve a response.

Good riddance. If it shuts up, that's fine by me.

Forcing a show of indifference, I sat up in bed. That's when my phone buzzed with a message.

"Guh......"

My breath caught the moment I read it. Right, she said yesterday she'd come by today too. I'd managed to send her away yesterday by wrapping myself in blankets and playing sick, but what now?

"Think... gotta think......"

My mind went blank. Lying in bed like yesterday wouldn't work forever. And I sure as hell couldn't just say, "Hey, I'm actually your brother," in this cute girl's body. There was no place to hide in the apartment, no logical explanation for this mess.

The devil's whisper. But it was also a savior's. It was the root of all this insanity, but it was the ‍‍‌‌⁠​​⁠‌​​‍​​‍​‌​⁠‍‌‍⁠⁠‌⁠‌‍‌‍‌‌‌‍⁠​‌​‌​‌‍‌‌‌⁠‍​​‌‍⁠‌‌⁠⁠only thing that could fix this absurd situation.

"Haa...... What the fuck do you even want?"

I replied indifferently, too drained to even get mad. I could guess the price it'd demand, the depraved conditions it'd impose, but with Yuna about to barge in—a very real terror—I was ready to pay anything.

"More bullshit......"

It was a pro at push-pull. It knew my desperation but still played it cool, stepping back just enough.

"Haa, fine...... What is it? Spit it out."

In the end, I had no choice. Swallowing my frustration, I surrendered to it. Whatever forced quest or price came next, it had to be better than Yuna seeing me like this right now.

"......What?"

What kind of nonsense was this? I stared at the system window in disbelief. Open the door? That meant facing Yuna in this cute girl's body. It was straight-up telling me to blow my cover, and calling it a reward.

I'd steeled myself for anything absurd, but this didn't even make sense.

"Hey, you said you'd help—this is your idea of help?"

Of course. I should've known better than to trust it. I clicked my tongue at myself. But even to my sharp retort, the window stayed silent.

Does it really want me to do this?

I eyed the flickering window cautiously. No failure penalty listed. But the reward was weird too. "Identity Reorganization" probably meant fixing my ID and stuff to match Irina's body...... It was useful right now, but what did that have to do with facing Yuna?

Reorganization...... Memory Reorganization?

Wait—did it mean messing with Yuna's memories? No way. After treating me like a toy, I wasn't letting it touch Yuna too.

"Hey, hold up! You planning to screw with Yuna?!"

I yelled at the window desperately. I didn't want to escape this by wiping or altering her memories to make her see me as her "big sister." To Yuna, I had to stay her pathetic—but always there—"big brother."

"......?"

Then what good was that reward? Before the doubt could fade, a sharp chime shattered the silence.

Ding-dong!

"Already?!"

I hadn't even decided yet! As if mocking my panic, a familiar voice came from outside.

"Oppa, you still sleeping? Should I just let myself in?"

Her actually opening the door herself would probably fail the quest. I squeezed my eyes shut.

Fine. Let's see how far this bastard's game goes.

In a fit of resignation, I bolted to the front door and grabbed the handle.

"Let yourself i—!"

Beep-beep-beep! The door lock chimed cheerfully, and the door slid open slowly. Through the gap, Yuna's round eyes met mine.

"Ah, Oppa...... wait, you're not? Who...... are you?"

Wariness clouded her face—she'd expected her brother to greet her. And so, the fateful moment arrived: me, Irina, face-to-face with Yuna.

"Heh, so ‍‍‌‌⁠​​⁠‌​​‍​​‍​‌​⁠‍‌‍⁠⁠‌⁠‌‍‌‍‌‌‌‍⁠​‌​‌​‌‍‌‌‌⁠‍​​‌‍⁠‌‌⁠⁠you're Irina?"

Yuna sat across from me on the living room sofa, eyes sparkling with curiosity. The suspicion from the doorway was gone; now she looked at me like a cute little sister she'd known forever.

"Y-Yes! That's me. Irina."

Cold sweat trickled down my back as I nodded awkwardly. No clue what the system had done, but Yuna didn't bat an eye at a total stranger girl in her brother's apartment.

"Hehe, nice to meet you. I'm Yuna. Yi Yuna."

She beamed and offered her hand. That innocent smile only twisted my heart more.

"N-Nice to meet you too, Yuna un...... ni."

Hey! What the hell happened? You said you wouldn't touch Yuna!

I shook her hand with a forced smile, but inwardly I was screaming at the system.

Allure Scent......?

I bit my lip, clueless about whatever that was.

"Anyway, greetings done...... Lina? Can I call you Lina? So, where's our oppa? I told him to stay put at home!"

My heart dropped at her question. The "real oppa" was sitting right in front of her in a pretty girl's body, and I had no idea ‍‍‌‌⁠​​⁠‌​​‍​​‍​‌​⁠‍‌‍⁠⁠‌⁠‌‍‌‍‌‌‌‍⁠​‌​‌​‌‍‌‌‌⁠‍​​‌‍⁠‌‌⁠⁠how to answer.

"Oh, uh...... Oppa had a sudden errand. He'll be back soon."

Irina, is that the best excuse you got?

I inwardly berated myself, sweating bullets, but Yuna bought it without question. Or rather, she was too distracted by me to care.

"Sudden errand? From the guy who never leaves the house? Wait...... no way!"

Yuna suddenly leaned in close. Her eyes rippled strangely, gleaming.

"Lina, are you...... dating our oppa?"

"Wha?! N-No! Absolutely not!"

I flailed in panic. But Yuna ignored me, sliding next to me and fiddling with the ends of my hair.

"Weird. No way he'd let a girl who's not his girlfriend hang at his place. Ah! Lina, date me instead then?"

"......?!"

I was speechless at her bombshell. Yuna gazed at me with eyes like she could devour me whole.

You bastard system! What did you do!!

Its cheeky response made my neck snap back. Yuna's hand slyly wrapped around my shoulder, brushing perilously close to my chest.

"Lina, why are you shaking? Does it...... feel good?"

"N-No! Yuna, wait. Let's calm down first......"

"Forget oppa. He's not good enough for you. I can make you way happier."

Her face buried into my neck. Her hot breath made me squeeze my eyes shut.

"Hng......"

A faint moan escaped me. Yuna nuzzled my soft neck skin, inhaling deeply like it was a drug.

"Haaah...... Ah, Lina. You smell so good."

"Eek......"

This couldn't go on. I grabbed her arms to push her away—

"Guhk......?!"

Something alien invaded my mind. A ruthless assault that made it impossible to stay myself. Strength drained from my hands. Even what I'd been thinking moments ‍‍‌‌⁠​​⁠‌​​‍​​‍​‌​⁠‍‌‍⁠⁠‌⁠‌‍‌‍‌‌‌‍⁠​‌​‌​‌‍‌‌‌⁠‍​​‌‍⁠‌‌⁠⁠ago slipped away.

"Hehe, Lina's hands are going limp~?"

Yuna's low whisper tickled my ear. Against my will, my lips moved.

"U-Unni......"

This is wrong! my head screamed. But why? I couldn't input thoughts—just output them one-way. My heart just urged me to accept my little sis—no, big sis's—touch.

Yuna's my unni......?

Was she? Drool dribbled from my slack mouth. Yuna smiled at the sight.

"Aw, drooling already? Feels that good, huh."

She wiped my chin with her finger, then licked it.

"Th-That's dirty......"

Even my protest came out as a weak moan.

"You know, Lina? Your saliva's super sweet~?"

"N-No wayyy......"

I strained to push her away, but it looked like nothing but coquettish whining. Thinking it cute, Yuna stroked my hair.

"Hweeh...... hweee......"

Just a pat on the head, and my mouth gaped idiotically. Each touch from "unni" melted my brain cells one by one.

"Lina, looks like you're ready now. Hehe."

Her gaze slid from my wet lips downward. My massive breasts twitched under the thin pajamas wherever her eyes lingered, asserting themselves. My nipples—hypersensitive from last night's "development"—poked through the fabric, trembling temptingly.

"Whoa, Lina. How do you have such big, pretty tits?"

"I-I don't knooow......"

Thinking was impossible now. My voice was completely melted.

Yuna's finger approached the first button of my pajamas slowly.

Then, a voice echoed from deep in a cave in my mind: ......No. Irina! Snap out of it! If that button comes undone, it's over!

I tilted my head. Why did it sound so distant? I should obey, but I couldn't. Didn't want to.

"Hweee......? Who's thaaat......"

"Huh? Lina, what'd you say?"

Yuna paused, tilting her head. My unfocused eyes stared into space as I mumbled.

"Someoneee...... saying nooo......"

"Someone? It's just us here. Listen to me, Lina."

"Y-Yeah......"

She smiled brightly and returned to the button. Pop. The first came undone. Cool air brushed my neck, and the voice in my head raged louder.

But as her finger moved to the second, it vanished.

"Hngut......!"

The heavy breast jiggled with the undone button. The slight motion sent shivers of pleasure through me.

Yuna, with two buttons gone, lost all restraint. Her hands undid the rest ruthlessly. Now, only the fabric flap remained, ready to fall anytime.

"U-Unni...... this is really baaad......"

Not refusal—a coquettish plea, arms crossed shyly over my chest like begging for more.

Swish. She brushed my slender arms aside and fully parted the pajamas.

Last night's self-torment left my nipples swollen pink, stiffly erect at the center of my heavy breasts. Yuna gazed in rapture.

"Wow, Lina, look at these. They're so mad, like someone teased them hard~?"

"Embawwassing......"

I covered my face, cheeks flushing. Yuna chuckled and flicked a nipple with her finger.

"Eek, hng?!"

Sharp pleasure shot from the tip, racing up my spine. Even that light touch arched my back like a bow.

"Lina, you're so sensitive. Hehe, not fully developed yet, huh? Unni'll train them good."

"Hng, ahn, hngg nooo...... haaak?!"

"No" was just agreement now. My mind blank, I spat meaningless denials by habit. My body trembled, awaiting her next touch.

"Hehe, look how eager you are. Too cute."

No more talk. Yuna's hands ravaged my breasts relentlessly.

"Haaak?! Hngguuk!! Heek, hngguuk......?!"

She gripped and shook them roughly.

"Uhyaaak?!!!"

Far more intense than solo play—my head snapped back. Drool poured from my mouth.

Squirt, spuuurt—

A flood burst from my lower half.

"Whoa, Lina. Just from some tit play, and you're gushing like this?"

"Heeeuk...... haaa......"

My pleasure-melted brain couldn't process her words.

"Never imagined you'd have such a sloppy pussy."

She smirked lewdly, then suddenly scooped me up. My system-sculpted frail body felt weightless in her arms. I floated up with a dumb "Uhhe?"

"Real playtime's on the bed, right?"

Her eyes gleamed mischievously. I couldn't resist as she tossed me onto the bed. My huge breasts bounced wildly on the soft mattress.

Straddling me, Yuna declared:

"Okay, Lina. That was just warm-up, got it?"