Domestic Violence

A few days ago my sister told our family that she has been being abused by her boyfriend. We got her and her 6month old daughter out of the house and to a friends. She said she didn't want to file a police report. Her boyfriend is a drug addict and since this all happened he has supposedly been sober and has taken a shot that blocks opiate highs. She thinks if he's sober that he won't abuse her. She said she's going home tonight. I'm scared for her safety but I don't know what to do. I've tried looking stuff up online and most links say that it's normal for women to go back to their abusers after leaving. One of the latest incidents involved him having her by the hair on her knees and threatening her with a knife. He broke his hand a few months ago by punching her in the back of the head (we just found out that's what really happened) and she said he even gave her bruises and a black eye while she was pregnant. After googling his name, I found out he has a history of this. He has done this to two other women. Again, I'm really worried for her life and her daughters. I don't know what to say though to convince her not to go back. She really thinks he will be different since he is sober. Any advice? Has anyone gone through this with a family member?