Maybe the DF'd Brother is happy and content with his life. I can relate to this. When my family gets involved in my life it causes nothing but stress and it is just not worth the trouble. Maybe the DF'd Brother is happy and wants to move on with his life.
I guess this is the reason I would approach hubby first, you can't risk a wronged man being rejected twice.
That said, I do hope he doesn't hold your husband at quite the same level of culpability as your parents in law. I guess your BIL has tried to build a life away from his family and pulled up the emotional drawbridge for his own self protection .
One thing I would make no apologies for though - straight taking alert here - if you have kids, for goodness sakes don't let the next generation have to go through this shunning cr@p. That really is the place where you have the ability to effect the most positive change.