OP my DB was in a very controlling relationship with a very dangerous woman (she has accused every one of her ex’s of violence and/or rape)
It was very difficult having a relationship with my DB as he wasn’t allowed a phone of his own or SM etc and so I would just try and be in touch with him as much as possible and would invite them both to things as much as possible (he’d never be allowed on his own).
The very few times I met him on my own, I would never slag her off.
It was very difficult but I had to tread lightly.
The worst thing you can do is slag them off because it pushes them closer together.
She punished him for taking his DD to A&E because he didn’t wait an hour for her to get home from work.
She broke it off and he accepted (which she hated), so then asked him to come to hers to get his stuff where she injured herself and phoned the police on him accusing him on rape and DV. He was arrested.
Fortunately there was evidence that she’d made it up so didn’t get charged (easily could have though) but he still took her back and said that she’s mentally unwell because of the trauma of her past relationships (all BS), so he actually felt sorry for her after falsely accusing him of rape and violence.
He stopped speaking to family and friends who would slag her off and so I was very careful to not do it.
Try and meet up with your brother as much as possible but do not slag the wife off or mention what you heard.
Focus on building the relationship with him (and maybe even her).
Good luck OP 💐