You told your sister's friends at school that you are well endowed and can last a really long time. Word gets around to her and she confronts you about it one day after school. She says there's NO WAY that her brother has a big cock. You swear to her that you do. You turn around and say that she probably couldn't get anyone hard or off. She ends up making a bet with you that she could get you off within 2 minutes for $100. You accept after a little persuasion. She rubs your cock through your pants and you get really hard, not expecting how really turned on your sister is getting you! You cum through your pants. She makes the bet higher. Your car and the money. You accept again knowing for sure you can control yourself this time. She rubs, spits, sucks your cock. She sees your face and knows you're about to cum. Then she tells you not to hold back and cum on her. You shoot immediately after she tells you. She's shocked and starts to laugh. She WON! She gets up and leaves in a mess. "See you at dinner." *wink