Hmm, my dBPDh has a very weird brother (and mother), too. He really dysregulates further when he talks to his family. What I have been told is that they reinforce his distorted reality. His family really has an "us" against them attitude. This extends to his mother's new boyfriend (from his brother's perspective) and any girlfriend's his brother has (from his mother's perspective). It makes it easier to realize that it has nothing to do with me and could be any other woman in my husband's life. How is the rest of his family?
Interesting. He has no other family at all. Both of his parents died when he was a small child & his brother is 7 years older. Here has spent a lot of his life trying to get acceptance from brother & brother has pretty much spent his life speaking like he cares one minute and abandoning him with his actions the next. Brother dumped him at a truck stop at 15 to hitchhike across 5 states without a dollar in his pocket.
But yes, he is still pretty loyal to brother.