C.K.
I've seen this situation as well. My older sister was not accepted into my father's family because she was not his biologically. My father adopted her when she was 8 years old, and they still never accepted her. She was treated much the same. My father finally went to them (my grandmother and aunt) and told them that she's his as much as I am, and they need to include her. They didn't. They never even tried. Things we're strained until my father's death, and I haven't seen/heard of/talked to anyone from his family since then. I didn't even attend my grandmother or aunt's funerals. I think it was brave of my father to stand up to them, but ultimately, just remember, it's up to them to bridge that gap. If they don't, then you may have to step back from them. If your husband isn't willing to do that, just make it clear that your children together might suffer to see their brother be left out. If you still can't resolve the issue, just let your husband go alone to family functions, with none of the kids. It's better to keep your immediate family intact and together as a "team", then to separate them for the sake of the older generation, at least IMO.