Help, my sister is choosing her cheating boyfriend over family..?

my sisters boyfriend is a cheater and a manipulator there are 12 girls so far and just recently another girl came up he has been talking to and having lunch with at work.they are engaged and my sister lives with him and his family so with this last time she said she is leaving him but i have been asking if she is leaving him and goin to move with me and i think she is changing her mind again and i can't take it anymore, he puts her down, takes food from her, tells her she is fat and he tells her she pushed him to do those things with girls that she is the reason. first it was her family that he would say was the reason but now they live with his family and its still happening i just dont see how she can let him do this to her. i have been supportive to her and trying to help her but its like nothing that he does hurts her enough to get up and walk away, so my question is if she dont leave him should i tell her i wont be around until he is gone and when he is gone ill be there or would that push her away?? nothing else has worked i just feel this might? thanks for your help!