Incest Seduction/Sit in your brother's lap
You really don't want to lose your bet and are willing to take drastic measures to make sure. You innocently slide a little closer to your brother and then bounce up off of the couch cushion in order to plant your butt on his thighs.
"The fuck...?" Zack mutters in confusion.
"I want to sit on you," you reply plainly, as if it's the most normal thing for you to do while naked, and scooch backwards into his lap. In doing so you also scooch up against the shaft of his deflating cock and push it back until it's pressed against his stomach.
"Yeah, and there's a thing called personal space. Especially when you're naked." Your brother gives you a light shove but you resist, not fighting so much as patiently waiting. It takes all your willpower and tenacity to outlast him when he doesn't want you, but once you wait out his frustrations he sighs and caves and willing shares his lap. You're elated, but of course that by itself isn't good enough because it's not what you're here for. After sliding back over the tops of his thighs you lift your ass just enough to freely rotate your pelvis backward and help position your proud bum cheeks right around his cock, like that is its new home. It's even more fun to think that Zack's dick is taking the place of the spatula he placed there only minutes earlier.
"See? I just want to cuddle," you whine. "We used to cuddle lots." Although true, you've never cuddled quite this... naked before. Simply flexing your cheeks gives you goosebumps knowing what rests between them. Your brother's smooth college cock looked impressive enough just hovering in the air from a distance, but now that you've sandwiched him with your ass he feels so much thicker than he already looked. Not that you're the most qualified person to judge your older brother's size, but he sure does remind you of daddy, and daddy stretched you so hard.
"No. You don't want to 'cuddle'. But I am trying to watch TV."
Zack grabs your waist to toss you off but you're already snaking yourself around him. You lock your feet behind his calves and slip your arms down either side of his torso, reaching around behind the small of his back. Now that you've found your position you are ready for a fight and he knows it. He tests this, gently, and doesn't move you an inch. He sighs and lets go, letting you win yet again. He at least does love you like a sister. To show your appreciation, you whip him square in the face with your ponytail with a sharp turn of your head then lean to one side to look him in the eye pointedly.
"Oh. Sorry. Am I in the way?" you ask with a naughty smirk.
With an almost weary glare, Zack sighs. "Yes."
"Can't you still see the TV? You're super tall and I'm real little. I mean I'm not even a highschooler yet, remember? So there's no way I can actually be in the way, right?" You lean back against his chest and rest your head on his shoulder. "I think you're fine. You should just pretend I'm not here. I'm just going to get comfortable."
You wiggle your bum to prove you're trying to get comfy. It's mere coincidence that you are zealously rubbing against Zack's cock in the process. Your brother is clearly annoyed but he's at least open to you antagonizing him, no different than him antagonizing you. Maybe a little different. You're pretty sure he could stand up if he wanted, even with you in his lap. He's no longer the wimpy teen he used to be when you were really little but even then, like now, he's way bigger than you. And it's so much fun to think that you're in control of him.
Your 'unintentional' wiggling quickly turns into rhythmic writhing, rolling your pelvis back and forth to surreptitiously stroke your brother's thick hard shaft. Strictly speaking you've done what you wanted to now; you got him at least as hard as he was, if not harder. But now that your big brother's cock is pressed against you at full attention, splitting your ass apart, you want to explore this. You want to tease him. You want to have fun. You start to squeeze more vigorously and roll your cheeks a bit faster. In short order you're all but outright masturbating your brother with your butt. Until...
"OW!" you shout, trying to keep your voice down. Your back arches all by itself and your feet plant themselves on the floor to push yourself up, your butt sliding up Zack's stomach.
"Does that hurt? But you're so horny? Don't you like it when I touch you?"
"Fucker!"
Zack sure is touching you, but not the way you want him to. He's grabbed both your nipples and yanked up on them, hard. It does hurt a lot but somehow also makes your pussy ache. You don't know what that means. It's mostly confusing. Slapping at his hands is all you can do to keep yourself from elbowing him in the face. But he doesn't give you enough time to try that anyway. He chuckles a little as he lets go and you rub your sore nips as you let yourself slide down his stomach again... Until you hit his dick.
Your legs tense up when the very tip of your brother's cock pokes at your lips. You both become deathly still when you realize what could happen here. But if it's just an accident... then it's no one's fault, right?
With a little wiggle you slide down an inch more. Part of you wants him to push right inside you but you're not that lucky. Either you're too tight or the angles are just wrong because he ends up popping forward between your vulva and glancing off of your clit. You shiver as you sit down the rest of the way, slightly disappointed, angling his erection forward slightly as your pussy rests against the base of his cock. Neither of you dare to mention how close that might have been. It even takes a second to get your indignation back.
"Geez... I'm just trying to relax and you wanna be a jerk. When did you turn into such a meanie?"
"When did you become a nudist?"
You whip your ponytail again, trying to hit him harder this time. "This morning."
"Yeah? Same." Zack grabs your ponytail after it hits him and gives your head a short jerk with one gentle yank. Your mouth pops open as your head tilts back and your thighs reflexively clench together. You're still soothing your nipples but they quickly stiffen beneath your fingers. You had no idea you would enjoy getting your hair pulled so much, not until this very moment. But closing your thighs brings your attention back to the thing sticking up between them.
"Yeah... Well... I'm not the one who is going to lose a bet. You're really a total perv, you know that right? Only pervs get hard looking at their sister. God, and you're still so hard."
Zack's dick is nestled right up against your crotch, tucked in tight by the innermost part of your thighs closing around it, the top of it just poking up into the air above your own lap. You carefully wiggle and lift your feet up onto the couch cushions on either side of Zack, splaying your lower legs out to either side so that his legs don't get in the way of your knees, ensuring you can keep your thighs tightly shut around him. Your latest wiggle also shows you how tantalizing it is to have something hard and warm pressed up against your pussy. Having his long length shift and glide over your lips makes you tingle.
"You're such a pain in the ass, you know that?" Zack grabs hold of your hips, possibly to keep you from wriggling around so much.
"Your obsession with my ass is what got us here!" you shoot back, proud of your own quick reply. "I can't even cuddle with you without your boner getting in the way. Wasn't it supposed to be gone by now?"
"You're making that pretty hard."
"What was that? I am making you hard?! I thought you were just flexing?"
"You're the worst..." groans Zack.
"No, you're the worst! And a liar too! Or am I still just your sister? Who wants their nakie sister on them, right? I bet you would hate it if your thing rubbed on me too, huh?"
You reach down between your thighs and gingerly pull back on the tip of his cock like a joystick. It's the weirdest thing, touching a boy's penis. Even though daddy's has been inside you, this is the first time you've touched one - a proper full-on bare cock - with your own hand. You had no idea when that would be, but you never thought it would be in your own living room, even if you had hoped somewhere deep down that it would be your brother's. It's hard to wrap your head around.
Meanwhile, Zack's bulbous head feels so strange to grab. It's so plump and warm and yet rigid. At first you are very careful to not move it too fast or pull on it too hard. It's even a little scary to touch it, like handling a loaded gun. But like a gun, touching one somehow makes you feel powerful. And once you realize you're not going to break it you get more confident.
"This will make it go away, right? If I rub my pussy all over it?" You immediately demonstrate, squeezing your butt and lifting yourself all of an inch up his shaft. Pressed right up against you, there's no way to keep your flowing excitement off of his cock and you promptly deposit that excitement on him. Moving back down makes you smear your own juices over your crotch which only excites you more. You slide and squeeze a few more times before taunting Zack further. "Because I'm so gross, right? 'Ew, my gross little sister's vagina is touching me! Ew!' Something like that?" There's no reaction from your sibling so you keep going. "I bet you are gay. All of your friends take out their dicks when they come to your dorm and you suck on all of them. I bet you dream about touching dicks, and... and licking..."
"You know what? You should keep talking. That'll kill this boner eventually."
With a frown you shut your mouth and keep moving. You kinda got lost while rubbing yourself against your brother's cock. All you can think about is dick, so that's all that came out. But you don't actually need to harass him anymore. You just need him to stay hard. That's all. And you truly do need a strong hard cock. You want him to be harder than he's ever been because it feels so fucking good to grind your clit against such a warm thick rod.
You clench your thighs tighter and pull him against you with an impassioned groan as you buck forward. You can feel him flex and strain against your mound but you want more. Maybe you can make him even bigger. You're sure going to try. Just a little faster, or a little harder. It sure helps your clit. The longer it goes on, the more you want. You're getting so close... to winning...
One moment you are blissfully grinding on your big brother's yummy cock, pulling the underside of his shaft to press him more firmly between your lips, and the next he's blasting his semen into your hands, his hot spunk instantly coating your palm and splashing back onto your crotch and thighs. It's so unexpected (despite all this) that it makes you stop everything mid buck, after which you feel the power behind every pulse of cum that ripples up his shaft, your vulva and clit still pressed against him, before it even hits your hands. You also notice both of you are panting and groaning with pleasure although you're very sure you haven't had an orgasm. You're just really worked up.
Both of Zack's hands are secured onto your waist now. He pushes himself up into you with little mini thrusts to help push through the mixed mess of liquid incest and finish his climax. But after a few more final thrusts he pushes you off rather abruptly, almost sending you tumbling to your hands and knees on the carpet. That would make an even bigger pickle; You're sure you wouldn't be able to explain away two cum stains imprinted into the carpet in the shape of your hands to dad, let alone mom. But with your brother's cum quickly trickling down your fingers, you clasp your hands together close to your chest to try to keep from dripping on anything and turn to face your brother.
"Well good job sis. I guess you never did intend on winning your bet, huh?"
"Huh? What? What do you mean?"
Zack looks at you like you're utterly daft, which you apparently are. "Seriously? Maybe you didn't mean to, but you just did me a favor. Good luck trying to keep this thing hard now. I'd love to see you try."
Shit! What have you done?! You've pushed him too far is what. Your brother's once hefty cock is quickly leaning to one side. But you can't lose! You got so close! Actually, you have no idea how much time you have left - you're not even wearing a sock, let alone a watch. But you have to try something... right?