My sister is dating an emotionally abusive guy!?

[Dessi Love]

I hardly ever see my sister anymore she isolated herself with him. He's 19 and my sister is 22.They've been together for 2 yrs now. Here is a little back story. We tried to give him a chance but he completely ruined it. It started with he told us(me and my other sister) we're not aloud to hang out with her because we're a bad influence on her. He told us one day that if we did not bring her home from a concert that hadn't even started yet he would break everything in their apartment and ruin her clothing. He TOTALED both vehicles that my mom gave to my sister. which he wouldn't let her drive even though he doesn't have a license. He has to have her facebook and twitter passwords. He is constantly breaking her phones. He disrespects ALL of our family. cussing us out anytime anyone tries helping her out. He cheated on my sister with our cousin but after all this I've realized there is nothing I can do because she refuses to leave him.

Now my issue is I refuse to associate with him, I won't speak to him, he isn't welcome to ANY family function and now my sister asks our family if he could come to our family Christmas because he wants to come. Tell me if I'm wrong. I told my mother ABSOLUTELY NOT but mother says it's only pushing her away but I don't want him to think he's even welcome to come or even think I want him to continue to date my sister. NEED SOME HELP!!!!! Any advice. Trying to get my mother to understand.