Dealing with abusive mother

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I've known about my boyfriend's mother tormenting him throughout his childhood, they have a pretty bad relationship and they often get in verbal and physical fights. (boyfriend lives with parents cuz that's our culture here)
Recently, he revealed to me that the reason why he hates his mom is because she sexually abused him when he was 8, and he didn't tell anyone about it when it happened.
He kicked his mother out of the house but I've a feeling she's gonna come back and I feel really angry about this whole situation.
My bf has a baby brother who used to sleep in the same bed as the mother (she's also an alcoholic and would drag him to bed everynight)
>Bottom line is, I think that my bf has failed to protect his brother. I feel like he should have at least taken a stance to protect his brother from that evil woman. I'm scared he will fail to protect our kids when we get married.
Also, I've never had experience dealing with this sort of situation, what should be done? How do I explain to them that this woman should be ostracized forever?