Prompt:
>Puberty hits your formally flat tomboyish sister like a freight train
>She's taking every chance possible to show you her new jiggly bits

Story:

>So tomboyish that she has more-or-less the same reaction you have, minus the erection; a "oh shit this is cool i wanna show this off to my brother" reaction
>So tomboyish that she rushes to your bedroom the moment she gets back home from bra shopping
>"Anon! Dude! Dude, check these things out!"
>Scootaloo drops her bag full of bras and yanks the neckhole of her top down past her chest.
>Her tits, bigger than you remember them being just a few months ago (n-not that you were looking at them or anything, b-baka) spring out.
>And boy howdy, are they ever tits!
>They're bigger than handfuls, and in the back of your head you can hear Scootaloo yammering about how mom says she's getting a growth spurt.
>The nipples are hard and thick, looking perfect for wrapping your lips around.
>The skin is smooth, there's no sag, andOHFUCK Scootaloo's pressing them into your face!
>"I know, right?! I wasn't fuckin' paying attention until this morning 'cuz mom's got small tits and she told me that granny has small tits, so I thought that mind would be tiny too. So I looked in the mirror this morning and I had that exact look on my face!"
>You try to push her away, but she's crawled into your lap and has her legs wrapped around your lower back
>"Nuh-uh! The is the COOLEST fucking thing ever, and if I don't show'em to you, I'm just gonna end up playin' with them in my bedroom all night!"
>She eventually lets go, but refuses to pull her shirt back up
>You and Scoots have always been close - really close - as siblings, and now she seems to think that this means she can start going around topless around you
>She's as fascinated with her new tits as you are
>This is a big part of her life, dammit, and she wants to share it with you
>And now that you're both maturing into adults, Scootaloo's curiosity has taken a... certain turn
>"So, 'Non, how often do you rub one off? Like, I do it almost every night. Ooh! Ooh, wanna do it together?! Dude, let's see who cums first!"

----

>Scootaloo enters your bedroom as she usually does, bursting through the door without any warning.
>And as luck would have it, you had JUST taken a break from studying so that you could finally tend to your aching erection.
>Stupid sexy sister, walking around the house with her tiddies out.
>Why doesn't mom do anything?
>And, as usual, Scootaloo isn't wearing a top.
>"Anon! I wanna talk to you about some-oh!"
>She spots your boner and grins
>When you move to put it away, your sister just waves her hands and says "no" until you stop.
>"Dude, this is actually kinda what I wanted to talk to you about!"
>She saunters over to your bed and flops down onto it next to you.
>"A bunch of the girls at school were talking about sex, right? Everyone my age is fuckin', y'know?"
>Scootaloo begins to idly play with one of her tits, squeezing it gently and fiddling with the nipple.
>A blush works its way onto her cheeks, but she continues without missing a beat.
>"And, like, the Sex Ed teacher was saying that you're supposed to be super careful and bone someone you love and trust for your first time, right?"
>...
>You have a bad-slash-good feeling about this.
>"And that got me thinking, dude."
>Scootaloo turns to face you, and she's got this big dumb grin on her face.
>"We should totally have sex, Anon!"
>She says this so loudly that you can hear "sex, Anon!" echoing down the hallway.
>God dammit, Scootaloo.
"I-"
>Scootaloo leaps on you excitedly.
>"Yeah! Oh, man, this is gonna be awesome! It's like, I love you lots, right?"
>D'aww.
>"And all those times you let me touch your boner when we rubbed off together made me realize that thing would feel SUPER good inside of me!"
"Maybe we shouldn't-"
>Scootaloo leaps to her feet and runs over to your bedroom door, which she slams shut and locks.
>"Do you have condoms?"
"N-"
>"Ugh, doesn't matter. I'll steal some morning-after pills from mom. Oh my gosh, this is so cool! Dude, we're should just do this all the time!"

----

>You are Anon, and this is the first time Scootaloo has suggested real actual sex.
>Sure, she shoved your face into her tits not too long ago.
>Sure, you masturbated in each other's presence.
>Sure, she touched your dick and you rubbed her off, and maybe it felt way better than when YOU do it.
>But sex?
>That's sorta pushing it.
"Scootaloo, I really don't think we sho-"
>"Ugh, c'moooon!" groans Scootaloo, rearing back in your lap so that she can sigh dramatically, "Don't be such a fuckin' stick-in-the-mud, dude!"
>She scoots back a bit so that she's sitting on your thighs rather than on your lap, which is something your boner appreciates.
>Wait, your boner?
>Scootaloo has this big, dumb grin on her face as she shoves her hand down the front of your pants and starts fishing out your dick.
>Oh, no
>You're horny now.
>"Yeah!" squeals Scootaloo as she pulls it out, "Yeah, yeah, yeah!"
>After a second, she starts to give you a clumsy handjob, not once taking her eyes off the head of your cock.
>And the more she rubs it, the bigger her dumb grin gets.
>"Yeah, dude, you love it when I do this, right?"
>She finally breaks eye contact with your dick and peers up at you.
>"C'mon, man," she says, elbowing you in the side, "Do me too!"
>Scootaloo pulls backwards and grabs the waistline of her loose shorts.
>She bought them a size larger than she needed because she and mom thought her hips were gonna get wider too now that her tits were hitting a growth spurt, but that hasn't happened.
>Yet, anyway.
>She rolls onto her back, rocking back and forth a few times as she impatiently drags them down her legs, whereupon she throws them into a corner of your room.
>With a final roll, Scootaloo rights herself and sits cross-legged on your bed, nude from the waist down.
>...and from the waist up too, frankly.
>Oh, god, your sister is completely naked and in front of you.
>Oh, Jesus, you never thought an erection could actually hurt.
>You yank your pants down your legs just so that you don't either snap your dick in half or rip a hole through the fabric, which makes your sister pat her thighs excitedly.
>"Fuckin' finally, Anon!" she cheers, "My teach has been goin' on about this thing for so long, dude, and I finally get to try it!"
>She blushes and bites her lower lip as she tries to beat your dick in a staring contest.
>"Fuck, man, this is so hot..."
>She drags one of her hands down the front of her flat belly.
>Her fingers part the purple hair between her legs, and you follow the path with your eyes.
>Your dick twitches when you see the slight sheen on the inside of her thighs
>Scootaloo begins to rub herself as she stares at your dick as though it'll disappear if she so much as blinks.
>This is too much for you.
>You have to relieve some of this pressure.
>You reach down and wrap your hand around your dick, pumping up and down the shaft.
>Scootaloo lets out a strangled moan as she jams a finger deep inside of herself.
>"F-Fuuuuuck~!"
>'Fuck' indeed, Scootaloo.
>'Fuck', indeed.
"Oh, Jesus," you moan, "Okay, m-maybe this ain't such a bad idea, Scoots."
>Scootaloo grins up at you and begins to play with one of her own tits.
>"I kn-know, right?" she replies breathlessly, "Y'wanna do it now?"
>...Scootaloo was always blunt.
>"C-Cuz, Miss Cheerilee made it pretty clear that sex happened when your dick goes inside of my pussy, Anon."
"W-Wow, my dick specifically?"
>Scootaloo laughs breathlessly.
>"Do you see any other guys hanging 'round me an' the Crusaders, Anon?"
>Thunderlane's brother, actually.
"What about-"
>"He does't count."
>Ouch.
>Poor kid.
>You wonder if-
>"Okay, wait! Wait, wait, wait!"
>Your sister pulls her hand away from her crotch, gesturing for you to stop fapping.
>The trail of fluid that connects her pussy and her fingers nearly makes you blow your load right then and there.
>"Fuckin' hell, Anon! Do you want me to squirt all over your sheets, or do you wanna bone me?"
>You want both, frankly.
>"C'mon, lemme just... aw, fuck it!"
>Scootaloo rushes forward and pushes you onto your back.
>Your head hits your pillow, and you realize how lucky you are not to have bashed your head against the wall.
"Scootaloo!" you yelp, "Jesus fuck, man, be careful! You can't juuuooohjesusmaryandjoseph"
>Scootaloo settles onto your lap, and your cock is currently hotdogging her plump pussy lips.
>A coating of girl-cum has made itself known on the underside of your shaft as she slowly rubs your dick up and down.
>"Oh my god, Anon, this is so cool! This feels weird!"
>She peers up at you, a look of wonder on her face.
>"Do you think sex'll feel even better, dude? 'Cuz if it does, then we should TOTALLY fuckin' bone whenever we get a chance, y'know?"
>You let out a breathy laugh and rests your hands on your sister's hips.
"There's only one way to find out, Scootaloo."
>Your sister grins at you and slides her pussy up towards the tip of your dick.
>"Dude?" she asks, hovering her entrance over the pre-dribbling head of your cock, "I knew I kept you around for a reason."
>And with that, she unceremoniously impales herself on you.
>You'd heard stories about a girl's first time.
>You heard they were supposed to bleed
>You heard it was supposed to hurt.
>But your sister...?
>"Oh man oh jeez oh man oh jeez oh man oh jeez-"
>YOUR SENTIMENTS EXACTLY
>Oh, fuck, how have you not blown your load already?!
>Jesus fuck it feels way different than your hand! O-Or even your sister's hand!
>It's smooth, and moist, and it's way hotter inside of her than you had expected.
>And that's just the tip.
>Inch by inch, you're slowly going deeper and deeper inside of Scootaloo.
>Scootaloo shudders as she slowly slides down your length as you get closer and closer to bottoming out inside of her.
>"Oh fuck me, this feels so-so-so..."
>Scootaloo hugs herself as she finally reaches the base of your cock.
>Shouldn't she be in pain or something?
>Gently, you reach out and cup your sister's cheek.
"You alright?" you ask, swallowing your mouthful of spit so that you don't drool all over yourself, "I-I think I heard that it's supposed to... y'know... hurt."
>Scootaloo bends forwards so that she can steal a kiss from you.
>"Nah, man," she giggles, "Mom was all-"
>Scootaloo puts on a falsetto and starts to wave her hands around.
>"-'Scootaloo! You're going to start being sexually active, so I bought you some dildos! You should try these out and see what you're comfortable with!' "
>She starts slide herself off of your dick, and you nearly see starts from all the pleasure being sent to your brain.
>Who knew you'd lose your virginity to your own sister?
>"A-And so I kinda broke my fuckin'... hyneman with that purple one you saw on my pillow two nights ago."
>Her...
>Her fucking what?
>Her hyneman?
>...
>Fucking Mythbusters.
>You don't last long.
>It's your first time, so frankly don't expect to.
>But if it's any consolation, your sister seems to be right behind you.
>As you grip Scootaloo's hips and thrust desperately into her as you unload yourself into her, Scootaloo lets out a loud moan and jerkily moves her hips around.
>A second later, she collapses onto your chest.
>You're completely spent.
>All you can hear is the sound of you and your sister panting, and the pounding of your heart in your ears.
>...
>You're gonna have to change your sheets before you go to bed tonight.
>"D-Dude..."
>Scootaloo begins to giggle, which turns into a belly-laugh.
>She pushes herself back up into a sitting position and grins down at you, looking just as excited as the day she shoved her tits into your face.
>...which was very recent.
>In your post-coital state, you can't quite recall if it was yesterday, or a week ago.
>Some significantly small amount of time, anyway.
>Small enough that you don't feel like you really missed anything going on between you and Scootaloo.
>"That..." she pants, "Was totally AWESOME! That was WAY better than how my teach said it would be! Oh man, we have to do that again! We just gotta! Do you think we have time to fuck in the morning, before school starts? Oh! Or! Or maybe we can do this again right after dinner? 'Cuz my gym teacher is always going on about exercise and burning off calories and-"
>>"Scootaloo!"
>Oh god, it's your mom.
>You're proper fucked now.
>>"Go to bed, already! You have school tomorrow; you can play with Anon later!"
>Scootaloo just sighs dramatically.
>"Fiiiiiine!"
>You hear footsteps depart from your door, and Scootaloo peers down sadly at your joined crotches.
>She starts to move her hips, but stops suddenly and shudders, slamming her eyes shut with displeasure.
>"Oh, wow. Okay, jeez. Maybe not right away. Holy fuck, is my pussy sensitive right now."
>She opens up one eye, and her smile returns.
>"That was like, the BEST, Anon."
>You can't help but grin back up at her.
"I know, right? You still wanna do this again later?"
>Scootaloo mock-slaps your bare, sweaty chest.
>"What happened to you being such a spoil-sport, Anon?"
>Easy.
"Sex happened."
>Scootaloo cackles and crawls off of your lap, leaving an unpleasant puddle of... fluids as your softening cock slips out of her.
>"I GUESS we gotta go to school tomorrow, Anon, but we have GOT to do this again tomorrow, right?"
>You lazily wave at her in response, your eyelids drooping.
>Is this normal?
>Are you supposed to feel this exhausted after having sex?
>You've read stories, but you never thought it would be this quick and hard-hitting.
>The last thing you remember before you fall asleep is an excited giggle and the sensation of your sister's lips on your cheek.

-----

>"Whoo!"
>Your sister bounces on your lap, her hands planted firmly on your shoulders.
>"Dude, my Sex Ed teach was RIGHT! This is amazing!"
>She leans down and steals a smooch from your lips, and you feel her lips part as she grins against your mouth.
>"Ugh, why didn't we do this sooner?!" she whines, kissing you again, "It's like, I knew that thing where I'd jack you off and you'd rub my pussy was super fun, right? And it felt really good!"
>She grabs one of her tits and squeezes it, and then moans.
>"Really, REALLY good!"
>Panting, she re-positions herself so that she can lay on your chest and still have your cock inside of her.
>"Like, WAY better than when I do it myself, y'know?"
>She's not wrong.
>When Scootaloo works you over, you cum way harder than when you touch yourself.
>And when she discovered oral sex online and asked a bunch of questions to her Sex Ed teacher?
>Brother, you were in heaven.
>And so was Scootaloo, frankly, since you learned how to eat a girl out just for her.
>"But this is even BETTER! Oh my god, Anon, those dildo-things mom bought for me don't even compare to this!"
>You can't believe this all started because Scootaloo thought it was so neat her tits were getting bigger that she barged into your room to show you.
>Speaking of...
>You lean up and grab one of your sister's swinging tits in your mouth and give that nipple a hard suck.
>Scootaloo gasps and her nose scrunches up in the cutest way possible.
>"O-Oh, man! I keep forgetting how much you love my tits, man!"
>You'd respond, but that would mean you'd need to stop sucking on your sister's nipple.
>So, you know, fuck that.
>Scootaloo's moaning starts to get louder, and you have no doubt that your mom can hear you.
>It's the weirdest thing, actually.
>She doesn't seem to care that you and Scootaloo rub each other off and - you guess now, anway - bone each other.
>As long as you fall asleep in your own beds, mom is fine with all this insestual stuff going on under her roof.
>"Oh, fuck! Oh fuck, oh fuck, oh fuck, oh fuck oh FUCK!"
>Scootaloo's insides tighten around your cock, and you decide to muffle your sister's moans with your mouth (which regrettably means you have to put down the tit)
>Scootaloo responds by wrapping her arms around your upper back and kissing you back.
>This entire thing sets you off, and you spew your load directly into your sister's pussy.
>It feels so good that it actually sets off a second, smaller orgasm in you.
>After you come down from your high, the two of you just stare at each other.
>It feels strangely intimate, compared to the rough-and-tumble sex-stuff the two of you have been doing these past few weeks.
>You can almost see something in her eyes, this time.
>Something you can't quite identify.
>Scootaloo finally pulls back and pats her belly, staring down at it contentedly.
>"Dude, I can't wait until the baby comes."
>You choke on your spit and damn-near have a heart attack.
"Wh-What?!"
>Your sister just quirks her head at you.
>"Yeah, man. You spat your goo inside of me, remember? That means a baby's happening now."
>Oh, god, did you just knock up your sister?
>Isn't that supposed to mean she'll give birth to a swamp monster?!
>Oh man oh jeez oh man oh jeez you aren't ready to become a father
>You can't do this!
>It doesn't matter if you and Scootaloo get along really well!
>It doesn't matter if you love her more than anything in the world, and probably more than a man should love his sister!
>It doesn't even matter if the thought of running away together and living in a far-off city so that you can hold hands and kiss and cuddle in public and nobody would ever know you were related makes your heart feel funny!
"O-Oh, man, okay," you pant, suddenly feeling ill, "I-I-I promise we'll make this work, okay? I-I-I won't run away, a-and I'll get a job, a-a-and I swear I'll love you forever, 'kay?!"
>"PFFFFT"
>You look up and are confused to see Scootaloo hiding her face behind her hands.
>After a second, she burst out laughing.
>"I got you!" she giggles, "You totally thought I was gonna have your baby, Anon!"
>You fucking what?
"...y-you're not?"
>"Nope! Dude, did you think I was lying when I said I was snatching mom's morning-after junk?"
>Oh, thank god.
>Oh, you're not gonna be a daddy.
>Oh, this means your life isn't over yet.
>...so why do you feel strangely disappointed?
>You mean, the thought of just living together with Scootaloo and being able to be openly affectionate around people who don't know you're related sounded... nice
>It sounded REALLY nice.
>Scootaloo breaks you out of your deep thoughts with a roll of her hips
>You realize you're still hard.
>"I betcha we can make each other cum like, 5 more times before mom yells at me to go to bed."
>...
"You're on."