Your girlfriend's sister is staying over after a night out, your GF has gone to bed and it's just the two of you now. She playfully teases you about your sex life, sisters tell each other everything after all and she's heard all about the big cock you're hiding in your pants ;) she comments on how the two of you have been eye-fucking each other all night, and lets you know that if it were her, there's no way she'd be in bed right now letting that cock go to waste! She wants to show you how much naughtier than her sister she can be, and you're powerless to resist, even with your girlfriend in the next room. You fuck her hard missionary and doggy style, while she struggles to keep her loud moans at bay. Let's hope your GF doesn't wake up and catch you! ;)