Your sister wants to go out to swim but she wants to remind you that mom and dad said to clean your rooms before they get back. You point out how skimpy her bikini is and she starts jumping and spinning around to prove it covers everything. She flashes her pussy quickly and covers it back up. You pretend you didn't see what she did, so she does it again but this times she forgets to cover back up. You're starring at your sisters pussy, hard as a rock, as she goes on about something irrelevant to you. She notices and suggests you take you dick out so you can play Don't Twitch. The rules are simple if she catches your dick throbbing or twitching for her, you have to stroke it 20 times while saying "i love my sisters butt." Once she's played with you enough, you get to cum on her butt.