Incest Seduction/Brush it off and put it on Kim
Ignoring your erection, you try to relax and place your arm behind your sister along the back of the couch, as if it's totally normal that your little sister just got you hard. There's several seconds of awkwardness as the tent in your pants becomes the elephant in the room. Kim looks at you repeatedly with a grin that she can't hold back, but keeps shifting her attention to your crotch.
"What're you looking at?" you ask flatly, honestly. "This? This is your fault. Besides, haven't you seen a boner before?"
Kim's grin fades. She faces forward but steals a long glance of your crotch. A moment of thought later she looks at you again and haltingly, hesitantly, replies, "...no."
"What?"
She opens her mouth, but has to look away before she can manage to eek out an answer. "No, I haven't seen one before, okay?" She sounds defensive, but in a more pitiful way than angry.
"Oh... really?" you ponder for a moment. You don't think she's had any boyfriends, but... "You used to just walk into my room all the time. You had to have seen at some point."
Kim drops her shoulders and guiltily shakes her head. There was tension before, but you seemed to be the only one feeling it. Now that you've accidentally called out her inexperience, Kim is the one feeling the pressure. She's looking down, picking at the couch, rubbing her legs, pretending to not look at your pants.
"Not even once?" you continue to prod.
"I dunno... I mean, I wasn't... I never meant to or anything. Like... Even if I did, I don't remember."
You have to call her out now "...bullshit. How many times did I catch you peeking into my room after bedtime? Or after I showered?"
"I... I wasn't..."
"And wasn't there that time you ran right up to my bed while I was mid stroke? I mean I yanked my blankets over pretty fast, but you just appeared out of nowhere. That was kinda weird."
"I'm not some perv."
"No, you just want to see your brother's dick."
Kim slumps down and looks around as if you're in a crowded room, lowering her voice to try to get you to lower yours. "No I don't!"
"You sure? 'Cuz I'm not so sure now." You grab your glans through your sweats and adjust your joystick side to side, inspecting it as if your pants weren't in the way. "I mean it is pretty nice. I don't blame you for being curious."
"Oh my god, Zack." Your sister adjusts her bra straps and straightens her shirt. She's full on blushing at this point but she's still stealing glances without moving her head even a single degree. "Now you are being weird."
"I'm just saying, you tried way harder than I realized to get a glimpse of this guy. I really never knew you wanted to see it so bad!"
"I just... was a stupid kid, okay? Even if I tried I never even saw anything. It's really not a-"
You interrupt your sister with a burst of a laugh. "Oh shit! You really wanted to! You did! You just admitted!"
"Okay! Jesus!" Crossing her arms, she finally turns to face you, brow furrowed and frustration boiling over. "Just, like, kill me already! Gawd... I don't know why you're making such a big friggin deal about it anyway."
"Alright, alright. You're right. I suppose it's not a big deal."
You've probably harassed your sister enough. You try to relax again, but your boner refuses to. The idea that your sister has actually wanted and even tried to see you naked before is weirdly keeping you stiff. There's an even more awkward silence as you continue to look at each other, with you fully hard, both of you waiting for the other to speak. You might have to ask dad to go out and buy a chainsaw so you can cut through this tension at some point.
"So...?" your sister starts expectantly, her face still fuming hot.
"So? So what?"
"So are..." your sister stops herself seemingly out of humiliation. She then channels Dawn, sighing with a huff and rolling her eyes. But, albeit with some difficulty, she does manage to finish her thought. "Are you going to show me or not?"
Is that what she was getting at? That letting her see her big brother's stiff cock shouldn't be a big deal? Kinda feels like a big deal. And yet it's not everyday that a girl covertly feels you up and then all but insists to see your cock. In fact that's probably more of a once-in-a-lifetime kinda thing.