Your sister catches you watching her rubbing her panties, and it doesn't end with her telling you to fuck off. She's really irritated about this. You have no right to be in her business.
You'll get to watch though, she says, and she can tease all day - but you're going to get a real SCARE just as your cock gets excited for something. She's going to randomly jump scare and laugh at you, and she's in total control of your cock now.