I'm living with a 16 year old and I need help?

This boy is going to cheat on your sister if you dont put a stop to it. Tell him if he doesnt control himself that you will tell your sister, if he loves her then he will stop. if he carries on with this girl from the studio then its obvious what you have to do. You cant side with this guy over your little sister, its not fair on her for all this to be going on behind her back. Do you really want to see her getting hurt?