What should I do, my sister and friends turned on me?

My sisters talking to this guy who used to be addicted to drugs, and supposedly broke out of a mental institution. Shes changed a lot since shes talked to him; by the way she acts around me and my friends, shes constantly texting him. Shes said many times before that shes not going to get involved with him and shes going to stop talking to him, but that has yet to happen. I have other friends coming up to me telling me that my closest friends and my sister were talking about me during their biology class. You can only imagine what you feel when you hear that your own sister did nothing to stick up for you. They were talking about how I won’t let my sister date this kid when I’m not stopping her from it. My sister was agreeing with all the rude things my friends were saying.All this boy is, is problems. I just don’t know anymore, what should I do ?