'A Bet's A Bet'
Your bratty sis knows you jerk off thinking about her... she tried telling you off after she caught you perving through the door, but you had enough of her attitude. After snatching her phone, you stuffed it down your pants... she jumped on you to get it back. Her shirt hitched up above her nipples in the scuffle, which only made you harder. As soon as her hand bumped your swelling cock, sis recoiled calling you a pervert. It was her fault for wiggling around, letting her boobs bounce out of her top, and for straddling you in the first place.

You bet she liked touching it secretly, the slut. And you also bet she wouldn't be able to guess the size of your dick, a wager she quickly took. If she was correct, you'd give her phone back and leave. But if she guessed wrong... she'd have to suck you off. Good thing you weren't all the way hard when she felt it... she'd guess wrong, you knew it. And you'd finally get your slutty little sis to give you a blow job... and more.