Thicker Than Water
Pairing: Stepsis!Natasha x Fem!Reader
Summary: Your stepsister has always been good at keeping secrets. What's one more between sisters?
Content Warning: Stepcest/fauxcest, loss of virginity, fingering (R receiving), use of vibrator (R receiving). Please be mindful this is not a healthy relationship between stepsisters!
Word count: 2.7k
A/N: "i'll write a fic completely different to stepdad!nat!" charlie said. apparently i can't help but drink the stepcest koolaid. anyway, this one was in the works for a while. i hope it's worth the wait.
The afternoon light filters weakly through the half-drawn blinds in Natasha’s room, casting long, thin shadows across the cluttered floor. Empty beer cans and discarded clothes are scattered across the floor, and a half-smoked joint lingers in an ashtray. The air smells like her, sharp perfume mixed with cigarette smoke and something deeper, warmer, like skin after a long night out.
Natasha is your new stepsister, the daughter of the man your mom married six months ago. She’s supposed to be family now, but nothing about Natasha feels very sisterly.
You’re curled up on the end of her bed with a pillow hugged to your chest, eyes fixed on the TV sitting on the dresser. Your mom and stepdad don’t let you have one in your own room, “too distracting from schoolwork”, but Natasha’s twenty-one, so she does whatever she wants. That includes inviting you in here when she’s bored and hungover, which happens more and more often.
Natasha’s sprawled beside you, one leg dangling off the bed in her tiny sleep shorts. She flips between channels lazily, the remote loose in her hand, like she couldn’t care less. “This movie’s shit,” she mutters. The flickering channels finally lands on a mindless action film. “But better than staring at your phone all day, right?”
You nod, glancing at her shyly. “Thanks for letting me sit in here. It’s nicer than the living room. Mom’s always hovering.” Your voice is quiet, shy, almost.
Natasha has always been the opposite. Her voice is brash, her tongue almost barbed whenever she talks to your parents, her heart full of secrets she keeps from them about where she goes at night and who she sees.
She snorts, tossing the remote aside. “Yeah, well, Dad’s the same with me. ‘Where are you, who are you with?’ Like I’m gonna tell him I’m at Tony’s again.”
She reaches for an already open can of beer on her nightstand. You watch as she takes a casual sip, then tips it towards you, offering. You shake your head quickly, and she rolls her eyes with a knowing smirk. “Good girl. Wouldn’t want to corrupt you.”
The comment hangs in the air, slightly edged. You shift a little closer on the bed, knees drawn to your chest. Natasha’s always oddly attractive in that way, her ability to make anyone at ease unparalleled.
For a while you both watch the screen in comfortable silence, explosions and flipping cars flashing across it. Natasha comments on the bad acting, mocking the dialogue until you laugh.
Eventually the movie drags, and she turns down the volume. Turning to you, she props her head on her hand. Her hair falls messily over her eyes, but her gaze is still intense as she studies you quietly. “You’re quiet today. What’s up with you? School sucks?”
A half-hearted shrug as you pick at a loose thread. “It’s fine. Just…” You glance over at your stepsister, who motions with a nod for you to continue. A lump in your throat emerges, but you press on.
“People are just weird,” you murmur. “Everyone’s talking about parties and dating and stuff. I don’t really fit in.” The admission slips out easier here, in her room with the door closed and the TV humming low.
Natasha’s smirk softens into something more curious, though the glint never fully leaves her eyes. “You got your eye on anyone? Some nerd who pops a boner when you walk by?”
Heat creeps up your neck. “No. Nothing like that. I’ve never even…” You trail off, biting your lip nervously.
“Never even what?” Natasha scoots closer, her bare knee brushing yours. The contact is casual, but her voice drops a notch. She’s interested now. “Come on, spill. We’re basically sisters. You can tell me shit you won’t tell your mom.”
You hesitate, your heart rate picking up. Natasha’s always been this way, abrasive and pushing boundaries. Being around her feels dangerous in a way that excites you. “I’ve never really done anything. With anyone. Kissing, dating…”
“So,” she starts, voice low and rough from last night’s hangover, and the smirk on her lips seems less than sisterly, “you really telling me you’ve never even kissed anyone? Eighteen and still a real virgin?”
Her green eyes rake over you, predatory in the dim light. There’s a thrill in them, something dark and wrong that makes heat crawl up your neck.
You shrug again, cheeks burning. “I don’t… I mean, it’s not a big deal. I’ve been busy studying. And all the guys at school are stupid.” Your voice comes out small, almost childlike. Timid is what Mom calls it. With Natasha, it feels like hiding from a storm.
Natasha laughs, a short, abrasive bark, and takes another swig from the can before offering it to you. You shake your head again. Shrugging and setting it aside, she scoots closer on the bed until her knee brushes yours.
“Bullshit. Bet you’ve thought about it. Late at night, yeah, under the covers? Someone else’s fingers inside you instead of your own.”
Her words hit like a spark against dry tinder. You squirm, pressing your thighs together without thinking. “Natasha…”
“What? We’re sisters now, right? Stepsisters, whatever. We can talk about this shit.” She leans in, breath warm against your ear, smelling of beer and mint gum. “Tell me the truth, little sis. You ever cum before? Like, really cum?”
Your heart hammers. The room feels smaller, the air thicker. “I… I don’t know. Maybe? It feels good sometimes, but I can’t keep going…”
You can’t even look at her. Your fingers twist the hem of your shirt, exposed under her gaze.
Natasha’s hand lands on your knee, heavy and warm, perhaps possessive. “That’s so fucking cute. A real virgin, not like those girls who take it up the ass and say they're still waiting for the right guy.” Her thumb strokes slow circles on your skin, inching higher under the edge of your skirt. Your heart thunders. “Bet you’d look so pretty cumming.”
Your cunt clenches wetly around nothing. You inhale sharply but you don’t pull away.
She’s wrong. This is wrong. She’s your stepfather’s daughter. Your sister.
“Natasha, we shouldn’t–” you start, voice barely a whisper.
“Shouldn’t what?” she cuts you off, shifting so she’s behind you now, her chest pressed to your back. Her arms wrap around your waist, her lips brush your neck. “Admit it. You’re wet right now, aren’t you? Just from me asking.”
You nod before you can stop yourself, a tiny, humiliated whimper escaping your throat. “A little.”
“Good girl.” The praise is rough, laced with something hungry. Her hand slides up your thigh, pushing your skirt higher. “Spread your legs for me. Just a bit. Let big sis feel.”
The word sends a guilty jolt through your stomach. You hesitate, then slowly obey, knees parting on the bed.
Her fingers trace the edge of your simple cotton panties, already lightly damp. She hums approval against your shoulder, nipping lightly, making you shiver. “Fuck, you’re almost dripping. Does your pussy know you’re supposed to be a virgin?”
“Natasha…” Your voice trembles, but you can’t help the way your hips twitch toward her hand. As shy as you are, the ache between your legs wins.
“Shh. Just feel it.” Natasha hooks a finger into your panties and tugs them aside. The warm bedroom air against your cunt makes you blush.
Her index and middle finger glide through your folds, slick and confident, finding your clit with ease and circling with lazy pressure. You gasp, back arching against her. It’s electric, better than your own novice touches in the dark. Her skin is warm, calloused slightly from whatever rough nights she spends out partying, and the contrast makes you shiver.
“See? Nice and easy,” she murmurs, voice dropping a note, more commanding. “Bet you’d squeeze my fingers so good.” You mewl gently as she circles, and then she presses one finger inside you slowly, knuckle-deep, and the mewl turns into a moan. Your cunt clenches around the intrusion, a stretch that borders on too much.
“Oh god,” you breathe, head falling back against her shoulder. All of your senses feel overloaded. The wet sound of her finger moving, the warmth of her body behind you, the faint scent of her sweat mixing with your arousal, all of it mixes into something dizzying.
“Another one,” she decides, and before you can tell her to stop, to slow down, she’s pushing in a second finger. Both curl inside you, stroking a spot that makes heat flicker in your belly. You squirm, legs shaking, moans spilling from your lips. “There we go. Feel that? That’s your sweet spot, baby. No boy’s ever gonna find it like I can.”
You bite your lip desperately, trying to stay quiet. Your mother and stepfather are both downstairs. “Natasha, what if someone hears? This is… we’re sisters–”
“Stepsisters,” she corrects, and her fingers thrust deeper, faster, like she’s proud to say it. Her thumb finds your clit, and she begins to rub firm circles that make your hips buck wildly. “And you’ll keep this secret, won’t you? Just like I keep all my shit from Dad and your mom. Our little secret. Say it.”
“I-I’ll keep it secret,” you gasp, voice breaking. Pleasure builds fast, a coiling snake low in your belly. It’s too intense. Your hips rock involuntarily, chasing her hand. “Natasha, it feels… weird. Like I have to pee. I can’t–”
She only laughs. Her hand doesn’t slow. In fact, she almost speeds up, flicking your clit with her thumb like she’s had years of experience.
“That’s not pee, dummy. That’s you about to cum. Let it happen. Don’t fight it. I want to feel you soak my fingers for your first time.” Her free arm tightens around your waist, holding you in place as you squirm harder.
The words are filthy, wrong, but they push you closer to the edge. Tears prick your eyes from the new, overwhelming sensation. “Come on, let go. Cum for me.”
Your thighs tremble violently. The pressure crests, that urgent, full feeling making you whimper and twist in her grip. “Natasha– please– it’s too much, I think I’m gonna–”
“Do it,” she groans, teeth finding your earlobe. “Cum. Now.”
Your first ever orgasm crashes over you like a wave, hard and shattering. Your whole body seizes, walls pulsing around her fingers as pleasure rips through you in waves. You cry out, muffled against her shoulder, squirming in her grasp, gasping your sister’s name.
Natasha keeps her fingers buried deep, stroking lazily through the aftershocks, milking every last twitch from you.
There’s a lazy grin on her lips that makes your stomach spin. “Remember, you can’t tell Mom or Dad. This stays between us.”
You nod weakly, still floating, cheeks flushed and heart racing.
The aftershocks are still rippling through you when Natasha slowly, finally, withdraws her fingers. You slump against her, breathing hard, thighs trembling and slick. Your mind spins. Guilt, pleasure, disbelief all tangle together.
Natasha lifts her hand between you, her fingers glistening with your release. She lets out a low, amused whistle. “Holy shit, look at the mess you made.” Her voice is teasing as she watches your flushed face. “Bet you didn’t know your body could do that, huh?”
You can’t speak. You just stare, heart pounding, as she brings her fingers to her mouth and licks them clean, one slow, deliberate swipe of her tongue after another. The sight makes your stomach flip.
This isn’t normal. This isn’t what sisters do, step or otherwise. The wrongness of it sits heavy in your chest, but the words to protest stay trapped in your throat. You only watch, both mesmerized and ashamed as she savors the taste of you.
“Mmm. Sweeter than I expected,” she murmurs. “It’s like a cheap porno. ‘Innocent stepsister gets corrupted.’”
A nervous, embarrassed laugh slips out of you before you can stop it. Your cheeks burn hotter.
Natasha’s grin widens. “Lay back on the bed, baby sis. I want you spread out for me.”
You hesitate, shy and squirmy, but her tone leaves little room for argument. Hesitantly, you shift backwards to lay down on her messy bed. Her quilt feels worn and slightly rough under your back. Your skirt is bunched around your waist, panties still pushed aside, and you instinctively try to close your legs.
“Uh-uh. Keep them open,” Natasha reprimands, and then she reaches into her bedside drawer. You try not to look too curious, but then she pulls out something small and sleek, and your eyes widen. It’s smooth, with a tapered tip, but before you can ask what it is, she clicks it on, and a low buzz fills the room.
“Natasha… I’ve never–”
“I know,” she cuts you off with a shit-eating grin, moving to crawl between your legs. Her free hand pushes your thighs wider, exposing you completely. “That’s what makes this fun. Relax. Our secret, remember?”
Her nails rake across your skin. The vibrator touches your inner thigh first, the buzzing sensation making you jolt.
Natasha’s grin turns into something almost studious. She slowly drags it upward, circling your oversensitive clit without pressing directly on it. You whimper, hips twitching away and then back again, torn between too much and not enough.
“Easy,” she soothes, but there’s laughter in her voice. “It’s just a toy. Breathe.”
She presses the tip against your entrance, and you try not to squirm at the brand new sensation, biting your lower lip hard. She's surprisingly patient, letting the vibration tease you open, and when your hips lift, only then does she slide it in, inch by inch.
The stretch is completely different from her fingers. It’s relentless, buzzing deep inside you. Your back arches sharply off the quilt.
“Oh–oh god,” you babble, hands fisting the sheets. “It’s– Natasha– fuck, I can feel it–”
Natasha laughs, loud and unrestrained, clearly delighted by your reaction. “Look at you. Already losing your mind and I’ve barely started. So fucking cute.” She twists the toy slowly, angling her wrist so the buzzing presses right against that sensitive spot you didn’t even know existed until today.
Your thighs shake uncontrollably. Whimpers and gasps spill from your lips, needy, foreign to your own ears. “It’s too much– Natasha, please, I can’t–”
“Yeah you can’t,” she grins, pressing the heel of her hand down on your lower belly to hold you in place while she fucks you with the vibrator. “Say it again. Tell me how good it feels.”
“So good,” you gasp, “oh, I’m– I feel like I’m gonna explode–” Your voice cracks on the last word, hips jerking wildly. Your cunt clenches around the toy, arousal practically dripping down your thighs.
The steady, intense buzz deep inside you, the slick-slick sound every time she fucks it in and out of your cunt, Natasha’s warm hand pinning you down, her laughter washing over you like she’s enjoying your reactions more than anything. It's all too much.
She clicks the vibrator higher.
You cry out, back bowing sharply. “Natasha–!”
“That’s it. Cum for me. Really fucking hard this time.”
The orgasm slams into you like a freight train, much harder than the first. Your entire body tenses like a tightly wound coil, and you sob with pleasure, thighs clamping around her hand while your walls thrum violently around the intrusion. Something gushes out of you, soaking the patchwork quilt beneath your hips. You babble incoherently, Natasha, please, too much, completely lost in the haze of your own pleasure.
Natasha keeps the vibrator buried deep through every brutal contraction, laughing the whole time. “Holy shit, listen to you. Making such a mess. My little sister.”
You ride the peak for what feels like forever, thighs twitching until the pleasure borders on painful. Only then does Natasha finally ease the toy out, clicking it off, and you flush at the sight of it glistening in the light.
She tosses it aside, and it bounces off the mattress and lands somewhere you can’t spot it. Slowly, a grin on her face, Natasha crawls up your body, pressing a surprisingly soft kiss to your forehead while you struggle to catch your breath beneath her.
“See?” she whispers, some sick sort of satisfaction in her voice. “Way better than doing it alone. And no one else ever has to know.”
You lie there boneless on her bed, the wrongness of everything settling back in like a heavy blanket. You hate that she’s right.
