Thanks, PF
I have asked her to stop. She has gotten better, but when she does talk about my uBPDsis and I ask her to stop she gets annoyed and says it it not fair that she has to censor herself and can't tell me about my uBPDsis and her kids. Then, I just change the subject.
I can relate to your "middleman" dynamic. My family is a collection of middlemen... . everyone talks about everyone to everyone else. I call it "palace intrigue" and I find it destructive, annoying and leads to guarding myself all the time. I suppose that is why direct and open communication and being my authentic self is so important to me in my friendships and relationship with my own family (not FOO).
I will keep setting that boundary with my mother and do my best to keep working on myself.
As far as Facebook goes, my uBPDsis has used the "defriend" button as punishment for those that displease her. I was defriended last year which was fine by me. Now I selectively block people who are mutual friends so that I don't have to see her in my feed when she comments on other people's posts and comments. I don't mean to cut them off, but if they are true friends, that doesn't happen because I have and offline friendship with them (i.e., talk on the phone, email, see each other)

Thanks again. I am very glad this site and the members are here. It has been a great support system for me.