Hello Kygirl and welcome
I'm sorry you are dealing with all the challenges with your stepdaughter. It sounds like you have set up some good boundaries, and having an impact on your granddaughter. Thank goodness she has you in your life.
I have a stepson who is an addict and has mental health issues (diagnosed with anxiety and depression issues). His mom is likely BPD. My husband and I have come to realize we can't change his mom, but it is difficult to watch the impact she has on SS22. She enables his drinking/using drugs, rewards him with gifts when he does as she asks, etc. But we have set up some good boundaries (he can't live with us unless he has been sober for an extended period, is in counselling, etc.) and that really helps.
What behaviors have you believing she has BPD?
We certainly have lots of information on this site, and the people on the
Parenting a son or daughter suffering from BPD board can provide you with ideas and support.
Please keep posting and let us know how we can help.