<human>: My sisters boyfriend of 1.5 years blows up sometimes and calls her all sorts of nasty things for trivial problems. He's convinced her that she is just not being attentive enough to his needs. He hates all her male friends and ignores her if she ever chooses to hang out with them, despite the fact she has been friends with them much longer than dating him.

He keeps promising her he'll change after each blowout but nothing has changed. This is her first relationship so she wants to marry him and can't imagine her life without him.

She has finally decided this isn't normal or healthy for a relationship and broke up with him. She's now a wreck and thinks she has made a mistake and that no one understands their loves and that he didn't blow up "that much" and it wasn't that bad. 

I just need advice on how to support her as she makes her decision about whether to get back together with him or not. I am heavily biased and I hate hearing her talk about how he was a great guy and we just don't understand, as does the rest of my family. My mom is now fighting with my sister because she is so angry that my sister is considering asking him to take her back.

 I need to be able to support her independently of my personal feelings. I want to be able to support my sister without attacking her choices and I need advice on how to do that.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: sister left abusive bf, is now wavering and wants to go back. How to support her while not attacking her?