Yeah, that's as confusing as it sounds.

BPDh has heard this before--a therapist mentioned BPD once and I've mentioned it as well. He's kind of textbook, but strongly refutes any diagnosis. "I just hate my job," he says.
Yeah, that doesn't explain the issues.
We're working with a therapist now who is functioning from that baseline understanding of dh as BPD but isn't saying it directly to him because as soon as definitions get bandied about, he flees. Over the years we've seen probably ten therapists at various levels, most of whom he charms utterly. As soon as he senses that he's the "identified patient," he's outta there. He's really good at casting me as the crazy one, despite the fact that he's been committed in the past. That was frightening--a suicidal gesture involving a knife that the kids witnessed. The police came, took him to the hospital, and once the psychiatrist there realized that BPDh WASN'T high or drunk, shrugged his shoulders and said, "hey, stop stressing so much." They were going to release him that next morning but I insisted they keep him for a couple of days in the hopes that FINALLY we'd make progress in getting him some help. But he's ever so charming, and they deduced nothing.
I've actually recorded arguments before, in part so that I can double check everything said and also so that I have backup for a therapist as necessary.
Anyway, it's a Dr. Jeckyll/Mr. Hyde denial situation and I am SOO glad a therapist he TRUSTS has finally seen through the facade and continues to work patiently without alienating him.
Hope that makes sense. I can hardly wrap my head around it sometimes.
As for the brother issue, yes BPDh is hurting very much over it. He's not in a good place emotionally, but he's been pretty calm and untriggered despite that fact. A huge part of me wants to take my BIL aside and implore him to just BE KIND, but I don't want to meddle either. I don't know... .Thanks for your support.