Your sister catches you staring at her tits one too many times and decides to take action. She tells you to keep jerking off while she watches - in fact, shes going to go one better and film it, while encouraging you. Once you finish she explains exactly why shes filming you stroking your hard cock - now she can hold this over your head, making you do whatever you want since you'll hardly want your folks knowing you were stroking it to your sis. She gloats about how she has one over you now and then saunters away.