Incest Seduction/Try to seduce him: Make another bet
This is the worst. You kinda do want to admit you're horny but you don't want to be accused of being it, especially with the story you keep telling about how it's just a bet. But the fact that your brother was willing to give you a bet of his own makes you want to turn it back on him so bad. You don't know how yet but you need to make another bet with him. For now...
"You're not being honest, you're being gross! Maybe... maybe I do like being naked, but it's just a bet!"
"I'm being gross? You let me put a spatula up your butt."
"Yeah? Well you wanted to! You're being a stupid brother!"
Zack stares at you for a moment then nods his head. "Yeah you're right. This is... I shouldn't have a stiffy right now... Sorry."
His reply throws you a little. Your indignation might have backfired. "Huh? What do you mean."
"Well, y'know. You are my sister. So you're right, that's a little gross."
"I'm not gross!"
"What? No! You're not. You're just my sister."
With a fiery scowl you plant your hands on either side of your waist to better display both your figure and your disapproval. "And?"
"And... I don't know what's wrong with me. You won your bet, okay? Now how long do I have to stay like this?" Zack shuffles against the counter anxiously pressing down on the base of his lengthy cock. It's as if he doesn't want to touch it full on for fear of making it worse.
"You don't think I'm pretty?"
"That's... of course I do."
"But I'm not as hot as any of those floozies at your school."
"What are you... I'm not saying that. And they're not floozies. Well, maybe some..." he admits with a grin, memories flashing by.
"You are saying that. You said a second ago he is honest but now you're saying I didn't get you hard."
"Well... yeah I guess I am saying that." His tone changes after realizing he's talking himself into a corner. "Nevermind. We're just playing around, right?"
"So then why do you have a boner!?"
"I dunno. Maybe I thought it'd be funny to smack you in the face. So I just, like, flexed it when you were down there."
"You flexed it?" You pause to look down and study his erection. "So... you're flexing it now?"
Zack shrugs his forced indifference. "I guess so."
You punch his arm which makes him outright grin, his amusement quickly bubbling to the surface. "Bullshit! You just don't want to admit your sister got you hard."
"Because you didn't! It's really just an accident or something. It'll go away in a minute."
This is perfect. "Wanna bet?"
He stares at you for a long time waiting for more smack talk. When it doesn't come he realizes you're serious. You can't hold down the smile creeping up your face as he stares you down. "...shit. What now?"
"I think you're lying. I think my 'stupid sister' body made you horny. I think you like looking at me." You extend a finger and repeatedly poke him in his pec which you accompany with a cocky smile.
"Okay, okay. But what's the bet?"
"You just have to lose your stiffy. I'll give you... 5 minutes."
"Uh..." Zack glances down with little confidence. Does he not think he can or does he not want to commit to a bet? "I dunno about that."
"10 minutes."
Zack raises his eyebrows in sincere surprise. He definitely wasn't interested at first but he's downright intrigued once it's clear that you really want this. "Really? That's it? I mean, I can just do nothing and win?"
"Well you can't go anywhere. You have to stay with me. Else how will I know when I win?" You're trying so hard to act natural and bottle up your excitement. The thought of actually forcing your stripped naked brother to spend even 10 minutes with you is giving you butterflies.
"Yeah? What are you trying to win anyway?"
"I dunno..." Your smile softens to a smirk before you look away girlishly. "I think when I win you should have to prove to me how hot you think I am."
Your brother scoffs. "Yeah? Well if I win I think you need to take a cold shower."
You casually drag the back of your hand up the side of your torso and then up your neck, doing your damnedest to look sexy. "Really? Am I that hot!?"
"You're not hot, you're in heat!"
With a fresh scowl you better understand what he meant. "Zack! Be serious! I know that's not what you want! I can see it! Unless you're secretly into guys or something?"
"I'm not gay, sis." Zack demonstrates his contempt of your accusation by walking away. You abandon the lone spatula still swimming in the sink and follow him out into the living room.
"Are you sure?" you poke him further. "I bet you've tried lots of stuff at college, right? How many dicks have you jerked before?"
Your brother rolls his eyes and sarcastically replies. "Just one."
"Yeah? What was his name?"
"Me! My own! Zachary! I haven't touched any other dicks! Jesus."
Zack takes a seat on the couch and you promptly sit next to him with just enough space between you to not make it awkward.
"Alright, if you say so. I just thought that since you're not interested in me at all, that, y'know, maybe..."
"You're thinking too much."
Sitting down suddenly changes the dynamic between you. Sitting usually allows people to relax but for the two of you in this moment it only adds to the tension. There's so much uncertainty in apprehension, but you're still so excited to be naked next to your nude brother.
You press him further before the silence gets too uncomfortable. "So then? Tell me! What do you want from me if you win?"
He steals a penetrating side glance of your body as he considers the question. "Well I do think you should sort yourself out. Even if I'm horny - for some reason - you're way more worked up than I am. So if anything... I'd want you to just bang it out, get it out of your system."
"Yeah? You want me to bang? You want to see me..." Then you remember. "Oh. I can't do... That's part of the bet with mom. I... can't touch myself."
"Wow. How much were you touching yourself for that to be part of the bet?"
"I wasn't! I just... Okay, maybe I wanted to at first, bu-"
"You're downright freaky sis."
"No I'm not!" You deny, hitting him again. It's not that he's wrong. You've been very quick to embrace the darkest corners of your sexuality this morning. You literally jumped on your daddy's dick. But you don't want anyone thinking you're some sex-starved freak.
"Well you at least like making weird bets. You're just going to have to risk losing one with Mom if you want to try to win this." Zack looks to be very amused but when he notices the worry in your expression he shrugs and points down at his lap. "I mean, it's not like I'm going to rat on you."
You try to bite the smile that hits you as you openly gaze down at your brother's still very present erection. You've been trying so hard to follow your mom's bet to the letter but you're not going to give up an opportunity to push your brother around because of it. The idea of adding even more risk to an already risky situation makes your butterflies flutter and your pussy tighten.
"Okay, fine. It won't be a problem anyway because I just won't lose! So it's a bet!"
You stare at each other for a moment, both of you waiting for the other to do something. You've never felt so much sexual tension in your life from just sitting on a couch. Your own bubbly nervous energy might be the main thing keeping Zack hard at this point.
"Uh... I'm going to watch some TV."
That's disappointing. You're not sure what you were expecting exactly. For the two of you to creepily stare at each other for the next 10 minutes?
"Yeah, well... you can't avoid me! I'm going to stay right here. Good luck losing your boner!"
"Whatever Kimmy. I'm not even that hard."
You're worried you might be right. Zack turns on the TV and tries to find something to kill the mood. He lands on an infomercial, some terrible pitch for a boring product, which he stares at with an awkward intensity. You only glance at the TV, instead fixating your gaze on your brother's hard-on. You are far less confident you can win this now that you realize Zack can so easily distract himself beside you. Not even a minute later you can see signs of failure, his cock beginning to droop forward. You're going to have to do something quickly if you want to win!