Should I stay out of it or say something to him?

My sister had my beautiful baby niece in January and broke up with her dad when he went to prison when my sister was a couple of months pregnant. He came out of prison and ever since he has been pressuring her to take him back.

She found out he's had 4 other children with different women and recently got another girl pregnant. He lies about everything and generally isn't a very nice person.

A couple of weeks ago he stayed the night (he lives far away and it's easier because there aren't many buses so he often can't get home) and a couple of things happened. He grabbed onto my sister's arm and went right up to her face. His excuse was "I didn't mean it."

Then my sister said that she heard him shouting at my niece (who is only 5 months old) and he said "shut the f-uck up" to her. When my sister walked in the room he acted all innocent. Something similar happened before when he "shoved" my niece to his girlfriend.

I'm worried that if he keeps going on at my sister asking to get back with her, that she will give up and get back with him. If that ever happened I would be worried for her and my niece.

Should I confront him? I did something a few months ago that went a bit far, so I don't want to do anything that will affect my relationship with my sister.