Ah, that's tough, RW. I feel for you a bit, now. My Mom died this year, too. I want to keep contact with my nieces and & nephew, but their mothers and their dad (& uncle, my brothers) are nuts!! Not hostile like your SIL seems to be, but still they are difficult. They stress me out. And I have family responsibilities where my brothers' inheritances are concerned, so I have to keep some contact, anyway.
It sounds like maybe your brother has decided to devote his energy to his wife & theirs, and all you can do is try to keep up regular contact, even if it's through SIL, hoping that things will get smoother in the future.