Thank you everyone for your responses, I knew I could count on you guys
My dad said some of the things you guys have said, about my brother eventually snapping out of it when he realises what state he's in... and I said to my dad, well surely someone should be there for him when he has nothing left, and my dad said no, he'll only learn if he's at rock bottom and is forced to start making amends. I can see the logic in that. If I'm there for him as soon as his wife leaves him, he'll have no reason to speak to my parents. If nobody's there for him, he'll be forced to think about his life and right the wrongs.
My mum told me to not think of him as my brother, in this situation, but just as any other 34 year old man, married for 5 years with no kids. Say they split up this year. The divorce will be lengthy and it'll take him a long time to get over it. They've been together for 13 years. How old's he going to be before he meets someone new? How old is he going to be before they start thinking about marriage and kids? His chances of being just like his friends, and having something in common with them to talk about when he's out of his bad marriage, are growing slimmer by the day. Soon, that door will close. Some of his friends have kids starting primary school soon. He's missing out on that experience. They don't hang out with other couples. They only hang out with her family.
Abusing my mum and suffocating my brother aside, I actually like my sister in law as a person. She's fun to be around and I've never seen her be horrible to him. Understandably, she was horrible to my mum because my brother told her the things my mum had said about her. That doesn't justify what she did, and I'm angry about what she did, but I get why she did it. From her view, my mother is being the mother in law from hell, nobody is good enough for her son, they've been together for 13 years and she's said these things etc... but the truth hurts. She doesn't recognise what my mum's saying as valid, and it's too easy for my brother to go along with it, rather that than deal with the real problem. Admitting she has an alcohol problem is admitting my mum was right and admitting he, and they, were wrong to react in the way they did.
But as my dad said, there will reach a day when he listens to his wife and her family say poisonous things about my mum and he'll snap. It might not be soon, but I don't doubt it'll happen eventually. My mum's not done anything horrible to my brother, he has no reason to hate her, not really. He'll get sick of hearing them call her names and think to himself, that's my mum. Don't talk about my mum like that.
I can only hope, anyway..
As for my best friend, we're hanging out on Saturday afternoon, supposedly... I'm half convinced he'll bail and I'm pretty sure he'll invite our other two close friends along too. I've spoken to them about how my best friend and I have been drifting apart, but I don't really want to have the conversation with other people there because I think that gives him an excuse to hide away from talking about it properly. I know that people drift apart and times change; but we've been inseparable for such a long time, we lived together for two years, I really didn't think this would ever happen to our friendship. It really sucks.