Drtakjh, our circumstances are remarkably similar. Glad you don’t read her ranting emails, and you recognize:
I have to work to remember that this is about HER, not me, and that her husband is under a lot of pressure to support her
I feel sorry for my uBPD sisters husband and son, as I have started to recognize that she also verbally abuses them. I can distance myself but they can’t. I am sorry your BIL has turned against your, that must hurt.
I find it helps to just focus on my mom, as you are focusing on both your parents. Our fathers sound a lot alike. My sister had an explosive relationship with my father and I believe she turns a lot of her anger towards him on me, since he is no longer with us. Now she talks about him like he is a saint. I hate the splitting, but hopefully when we are able to recognize and separate our sisters distortions from reality, it helps us keep our energy and attention focused on our caregiver role and ourselves. Your parents are lucky to have you.