My brother opened up to me. (Long post)

Ha

My brother has been irritable lately, really just plain angry. (He's 12). He's at our house at least half of the time. Today he has let me in on a few years of stressors at home (with his Mother) and he opened up to me about how people saying mean things bothered him and how he's unhappy with his Mother. How she doesn't pick good boyfriends, how she allows to much to happen and lies to him. My brother told me how our Mother's ex husband called me a fat cunt. (I don't think he knows what that means, but I did tell him it wasn't a nice word) and our Mother didn't say anything to him when he did. He also told me how our Mother's new boyfriend called my husband a dick and that upset my brother. He loves my husband like a brother himself and they have a great relationship. I'm a little lost on what to do. I talked to my brother a bit about how it's not our job to punish people, it's God's job and that it isn't right what some people do & say but there isn't a lot to do. But now I'm mildly annoyed with my Mother. I've been upset with her since her and her cousin started talking shit to my husband when I walked away, tried intimidating him and calling him a cunt. When I was in the hospital with sepsis, she also blamed that on him. I think this is the end of the line with her. What would you do?