I have a younger brother with BPD who doesn’t believe anything is wrong with him.
My BPD brother is the same.
His whole life people have suggested he needs anger management and that makes him ... angry. He does nothing to change.
To be fair, it's hard to accept something is wrong with us. For someone with BPD who lacks a clear sense of self, it's that much harder. There is no real self to take responsibility. Lacking a self means there are no boundaries. That makes it hard to tell where the source of anger or hurt or sadness or fear is coming from so pwBPD locate it elsewhere, usually on people close to them.
Is there something about the inheritance that needs to be discussed? He may be drawing you into a conversation that doesn't need to happen.
The people I've known in my life with BPD often use topics of interest to others knowing it's a way to engage.
I'm low contact with my BPD sibling. When we are in the same space together I tend to use non-verbal boundaries since they are less likely to stir up conflict. If I disagree with something he's saying and there is no consequence for me, I acknowledge (versus agree) and move on.
For example, my brother does not want my 80-year-old mother to drive. He's tried to enlist me in a caper to prevent her from getting her license renewed. While I agree she is in no shape to drive, I am not prepared to get involved in the issue. I don't say no, but I don't say yes either. It isn't a matter of not caring, it's a matter of figuring out ways to be effective and being realistic about what isn't.
It isn't effective to try and reason with him.
I find validating questions are the most helpful. I ask him what he thinks, I express some curiosity (trying to not get wound up, which is hard), I nod my head, I say Huh, yeah, hmm, interesting when he says something nuts. If he presses me, I have a handful of things I try to say to buy me time. He likes to bring up failures in my life, so I say things like, "Yeah, that's yesterday's chapter" and try to move on.
Do you need your brother's agreement on anything in particular when it comes to your inheritance?