That's great insight and advise livednlearned.
I would look into a therapist that might be able to help you zero in on some issues that your step daughter has experienced so that those feelings can be validated. So many books about having a mom with BPD and what the kids go through. There are ways to correct it or redirect so they don't suffer the long term effects of their childhood experiences.
Like with my step daughter, she told us that her mom gets jealous if she wants to call us or come to our house. Her mom will get mad and yell and say "why don't you just go live with them then." We just validated her and told her there is nothing wrong with her wanting to call us or come over on mom's week and nothing wrong with her wanting to go see her mom on our weeks. We live close enough to one another that we should be able to be flexible with that and we understand how she misses mom/dad when she is with the other parent. Then we just gently explained that this is an adult issue that we adults need to work on to make mommy more comfortable with it and promised her we would keep trying.
She seemed to feel reassured and it took the pressure off of her. She talks about the differences she sees. Like she says to me that I don't get mad, I just talk. I stay calm and always know what to say. She has told me I'm really good with people even when I don't like what they are doing. Warms my heart but my point is that they do see and feel the difference. They do recognize the emotional instability... .I have also been there when her mom starts a yelling match at dad. I have pulled her aside and told her to let mom and dad deal with this, you and I are going to go get some ice cream and I walk her away. These are not issues that a 9 year old should feel like she has to fix or feel in the middle of. Then when it's over, I let her know, "see they figured it out." At first she didn't want to leave because she felt the need to protect her parents from each other. Now she almost grins and comes running to me. Luckily there is a 3rd adult in the situation that can protect her... .her mom's boyfriend is like a puppet. He just stands there... .or sits there... .weird he won't get involved. It's kind of hard for my husband to remove his daughter from it when mom just runs after him. So we divide and conquer, he deals with his ex and I distract the little one.
Fun stuff right?
Bunny