@tryingtothinkofaclevername sorry to hear your brother may be going through that. Without wishing to hijack the OP's thread, I did want to reply because if abuse is happening to your brother, he will be in a very lonely place right now. In my case, I eventually told my family some of what was happening after they'd seen her explode a couple of times, and figured out that I wasn't in a good way. Ultimately, it ended the second time she assaulted me - my family were the ones who called the Police.
What you can do is a tough question. Approach with caution. Push too hard too quickly, and he may retreat from you, because talking about it will feel dangerous for him. I'd suggest gentle questions, no judgement, and simply ensuring he knows you are always there. Even when it feels like he's pushing you away.....be there. Because he's probably doing it under duress, or because he just can't find a better way to avoid triggering abusive behaviour from his wife, and he's desperate to make that marriage work (decent men don't walk away, right? They stay and work at it. And surely it can't be abuse - after all he's a man, and we all know abuse is something that men perpetrate - it's not something they're victims of. Right? All of that will be going on inside his head, and he probably doubts his own interpretation of what is happening - especially if it's not physical yet).
Finally, if there is any violence, don't hesitate to call the Police for him. That's not something I would ever have done. But when somebody else did it, it finally allowed me to see that it was okay to say this wasn't okay. And to walk away. Unfortunately, emotional abuse often escalates to physical abuse....So just keep your eyes peeled.
I wish you and your brother the best. Abuse is a sickening crime - but please don't judge him for trying to stick it out, or take it personally when he withdraws. He's just trying to survive and do the right thing.