Hi mermaidinjeans!

Welcome! You'll find a lot of us here who can relate to what you're going through--there are many of us with parents and grandparents with BPD who have had similar experiences.
My parents believe I have an obligation to allow her back into my life. After some long honest conversations, my mother is much better about it, but not my father. And my father is a big part of the reason I hate my grandmother (yes, I hate her, it doesn’t consume me or anything but it’s the truth). She was extremely emotionally and physically abusive to him and his siblings.
I presume that this is your father's mother. Are your feelings towards her based purely on how she treated him, or based on your own experiences with her?
I feel resentful about the guilt trips, but I also understand where my father is coming from. I wish I could talk to him about it without him getting defensive or passive-aggressive.
How has this affected your relationship with your father? You mentioned that he gets defensive or passive-aggressive when you talk to him. Does he seem respectful of your needs and boundaries otherwise?
Having an ex show up in your life after some time can be pretty jarring too. Do you see any similarities in what's going on with your family and your feelings and experiences concerning your ex?
Welcome again--there's a whole community here to support you.
-GG