Your sisters friend let's her in on a little secret. Guys can't cum if they aren't awake. She's not sure that's true so she checks in with you, you're her brother. You'd tell her the truth right. In her rant she let's slip to you its her friend and her brother who have been hooking up. This gives you an idea. Of course that's true you tell her. She "sneaks" into your room later that night to test the theory and finally get to feel an orgasm with a cock inside her. You are awake. Lucky the room is dark enough to watch her without her seeing. She rubs you, sucks you and finally hops on your cock. Riding you in cowgirl (dildo) and revers (dildo) until you silently cum inside her. She's surprised at how 'creamy her orgasm is' completely unaware. A few weeks later you see her sneaking out of the bathroom with a positive pregnancy test completely confused as to how this happened. She's never been cum in before... right?