Well, it's hard to say who is isolating who when it comes to your brother and his wife. His wife may have expressed that she feels intimidated and uncomfortable around his family so he has made a decision to distance because of that. It could be that his wife has social anxiety and he may be just trying to distance because of that too. All you can do at this point is just try to respect these boundaries he has put up even though it seems odd to you. You say he had a difficult time in the romance area so he may be insecure when it comes to his wife and he doesn't want anything to threaten this relationship he has with her.