Yes it hurts bad not having the support of my own mother. This isn't abnormal though it has been this way as long as I can remember.
I've read a lot of the articles on dealing with her with empathy. I've also read a small library of books on how to deal with a BPD.
Her own father and stepmother are telling me not to let her have the kids. I'm just going to have to fort up here at the house with the kids till she leaves town on Sunday night. She has threatened to burn down the house so I don't have a place for the kids. Oh crazy how I hate you.
Stay strong, man. I know this is a hard time, but you'll get through it. She's getting frustrated because you're standing up for yourself. Don't fold.
Make sure you're documenting these threats.