Hi Unipas987,
You have your hands full! I can relate to parts of your story so well: blending a family with a BPD child, and also going to school while raising a child. Both of those are a handful, not to mention you have some grand babies in the picture.
This caught my attention:
Her dad keeps getting angry at me for worrying and said he wishes I would just shut up and realise it's got nothing to do with me, the thing is it will.
There is
a concept called triangulation that might help.
I have found (being in the step role) that first priority is my own self care. You have school to focus on and your D9, those two priorities come first. Everything else must come second, as uncomfortable as it may feel.
Don't let BPD daughter's problems become your problems. Your H is telling you it's his problem, so as much as possible, let it be his to solve.
After self-care comes radical acceptance and rock solid boundaries. I can see how it will be hard to care for one grand child, and not the other. See if you can sort through what works with the one grand child, why that caregiving is acceptable. Does that step daughter give you sufficient time, does she seem grateful, is the routine consistent, etc. Then think about how you can apply those boundaries to your other SD and (I know it's hard) hold her to those boundaries. If she cannot ask in a polite way, then no care giving.
All of this is much easier if you have a supportive spouse. Maybe he is stressed by the situation and can't articulate it that way, and it's the stress that is showing up in his words and actions. It's also possible that he wants it both ways -- not letting you get involved but wanting you to absorb all the conflict.
In any relationship with a BPD sufferer, you have to have strong boundaries so that you are not caught in the undertow. This is even more true when it's a step child, in my experience.
Last summer, I kept waiting for my SO to fix things with his uBPD D20. I feel fairly skilled at communicating in healthy ways, even so, it mainly made him miserable and not a whole lot changed. It wasn't until last winter when I started to assert boundaries directly (and skillfully) that SO stepped up and started to carry his load. I think it's because he knows deep down that the weak boundaries were inappropriate and unhealthy. I think he also got jiggled out of his usual role when I stopped talking about the problem and started taking care of myself directly. He had no one to push against and found that the challenges were ending up in his lap, not mine.
Glad you found the site
You are not alone.
LnL