M.L.
Well, you said her complaints are almost all valid. Sounds like she has reason to be upset in that case and really needs to vent, perhaps she feels she has noone else to talk to. Since I don't know what her complaints are, I don't know the severity of the situation.
I would tell her, "I really think the best thing you can do is talk to a therapist, perhaps even relationship counseling if you want things to change. I feel for you going through all this, but this is my husband's brother, and my husband and I share everything. At this point, I would rather stay out of any discussions on relationship problems, though I do hope you get help and resolutions with them."