BPDFamily.com

I’ve noticed over the years that my BPD wife gets fixated on one person and just goes nuts over something they’ve done.  Sometimes the “offending” person was legitimately being mean towards her - other times it’s just perceived.  Either way this seems to be a common trait.  One year it was my oldest son - one year it was her mother - one year it was her sister.  Is this common in BPD folks?  It looks like its about to be my turn…

Yes, I think it's fairly common. There's some good discussion of it here (https://bpdfamily.com/message_board/index.php?topic=94786.0).

Google "Borderline Favorite Person" for in depth articles on this topic.

Normally it is a romantic partner; however, it can be anyone, and it can change from person to person. 

The Karpman triangle dynamics ( lots of info on Google and also a topic on this site) helped me to understand this. My BPD mother seems to perceive herself from victim perspective. This is a way to avoid feelings of shame and accountability. My father (and other family members at times) took rescuer position.

For this to happen, there needs to be a "persecutor" and that role can be filled with several situations or people. So, there needs to be someone ( or something ) to focus on who has offended her or has failed to meet her needs in some way.

You mentioned in your other thread that your older children are estranged from their BPD mother and she seems to not know why that is. With my BPD mother, I think at some times she knows exactly what she is doing when she's being cruel. At other times, I think she feels so much like a victim, she feels her behavior is justified.
 
I think I can speak for many of us on the thread for children of a BPD mother that- estrangement is not an easy choice or situation. Some have cut contact completely and others- I am one of them, have stayed in contact but maintained an emotional distance for our own well being. The Karpman triangle dynamics played out in my family, and this included if BPD mother was angry at me, she'd enlist my father as her "rescuer". At some point, we need to have a safe emotional distance and yet, it's also sad to think that this distance has been sad and puzzling to my mother, if that is the case.

It's encouraging that you have seen your role in "normalizing" this situation. While you don't want to triangulate with your kids (explaining that their mother has a mental disorder is probably better done with a therapist), if a child doesn't understand why their mother behaves the way she does, they may assume it's their fault.  Kids of any age want their parents' approval and unconditional love and you have the ability to give them this, even if they mother isn't able to.