Hi torturedsib23 and welcome. You'll definitely find others here who are supporting an aging parent while coping with other family members wBPD. Not an easy combination.
After experiencing his hostile and excessive emails and texts, it's smart of you to decide for yourself not to meet with him alone. It sounds like your husband is supportive of you in all this? Does your H understand some about PDs?
my older brother and I are named as co-POA (medical and financial)
Are there any areas where one of you is the "tiebreaker" (i.e., even though both of you are co-POA, are there some places where you can just move ahead with decisions on your own, or does everything have to be by agreement)?
We disagree about what to do with some of her assets, and he's been firing irrational, hostile texts and emails left and right.
Are there any areas where the two of you do agree on what to do?
Who is he sending these emails/texts to -- just you, other family members, attorneys, other...?
is there any advice about how to interact with him (given I can't just walk away) and protect my nuclear family from his attacks?
Sounds like he's expanding his attacks away from you and on to your family -- is that your husband, kids, more family members? How have they responded to the attacks?
...
While it may not be an easy or comfortable path, there are ways forward to not let a sibling with BPD totally run the show. Do settle in here, take a look at some other threads, and we'll wait to hear back from you whenever works best for you.
Glad you're here;
kells76