Hello everyone, sorry in advance for the long text and any mistakes as English isn't my first language.

I have come to a breaking point with my brother after his ex-wife called me crying the other night. I cut ties with him entirely almost two years ago because he's extremely abusive, manipulative, racist, homophobic, mysogynistic,... you name it. He's very loud and violent and is alcoholic to the point of ruining every family event. One Christmas, he beat up our other brother (35M) and punched me when I tried to pry him away from him even though I was 16. Some people in my family have completely stopped inviting or visiting him but most people even if they can't stand him just want to avoid drama and will simply get along with it. My parents completely enable him, pointing me as guilty for cutting ties with him and not wanting to "make an effort". Which gets me to the point - his wife recently left him after a very complicated 25 years long relationship, during 10 of those years he cheated on her with 200 women, and when she found out he asked her to marry him to "fix things up". She got pregnant again not too long after that and he quickly became very emotionally abusive again, insulting her, not helping with any chores, etc. Most of the time he's just drunk and high and screaming to himself about whatever conspiracy he's on in the moment. Well she called me crying the other night saying that she felt she was about to lose it entirely because he's been following her, coming to her workplace, waiting at the bottom of her apartment (she never gave him the address but he hacked onto every of her social medias and emails), spamming her with messages every night to insult her, threaten her to come over, etc. He's also completely manipulated their daughter (14F) and is developing a borderline incestuous relationship with her ; telling everyone they're fusional, taking her to very expensive holidays, having her do all of the chores his ex-wife used to do including cooking, doing laundry, etc. As a result, she doesn't want to see her mom anymore. On the other hand, their 4 year old son doesn't want to see his dad and keeps repeating her the insults he calls his mom. He also changed the locks to their house (which she still owns, as they haven't divorced yet), stole her car for a couple weeks, sued her for domestic violence after she slapped him for groping her. It's also worth noting that he first tried to prevent her from leaving him by holding her car keys hostage (they live in a small village without any public transportation) to the point where she had to call the cops. He's been trying to make this event play in his direction by calling her crazy and trying to get her into a mental hospital, and I think he's actually doing all of this just to get custody. I think y'all get the picture, he's basically one of the worst persons I know. My mom completely takes his side, saying that he's just feeling bad and not accepting the divorce, that "all men cheat" and that she "shouldn't be holding grudge when she agreed to marry him". My dad doesn't say anything because he's the classic emotionally unavailable dad. The point is, I'm getting scared for my sister in law. I've known her my whole life and basically consider her my sister at this point. She's genuinely scared for her life and I don't know what to do for her. I told her I'd try to talk some sense into my parents but my mom sides with him and basically no one wants to talk back to my brother because he'll become violent. He asked everyone to block his ex wife and starts screaming if you mention her. If anyone has advice, thanks in advance.

Tl;Dr my older brother (40) is abusive towards everyone and his ex-wife in particular. He's stalking her, threatening her and using the fact that everyone's too scared of him to act like an asshole to the point of actually scaring her.