Oh wow. Are you me?

I have a BPD step daughter (24), too, who has a BPD mom. It's been quite a journey.
In my experience, the main issue in these BPD stepchild triangles is your relationship with H and how you work with him.
BPD flourishes by generating drama triangles (rescuer/persecutor/victim) and a stepmom/father triangle can be the gift that keeps on giving.
The goal is to get out of the triangle and not allow her to create a wedge between you and H. I found I became ... almost a coach to H, if that makes sense.
Because your H was married to someone with BPD, he is probably prone to rescuing women in victim roles and SD25 is presenting herself as the victim because that works with dad. H will need you to model healthy behaviors here because this relationship probably strains him in ways that he struggles to understand.
When you have a stepdaughter with BPD and a husband with codependent tendencies, the boundaries are going to get tested. The bad boundaries will start with SD25 and H will extend them.
Don't feel bad about having boundaries when it comes to H and what he wants on behalf of SD24. Those boundaries are really important.
I always like solutions that put accountability back on the pwBPD. "I care about our relationship and I'm glad you're letting your dad know this is important to you. My recommendation is having someone skilled in facilitation help us reach a mutual understanding so we can get closure on this together. Let me know if you have someone in mind or if you'd like me to help your dad find someone."
Would something like that work?
My experience with pwBPD is that being accountable for arriving at reasonable solutions is too much sunlight. She'll sense a boundary and have to do a reset. When H comes back with, "SD25 says you're saying she's a mental case" or "SD25 is enraged you won't apologize and want to get a shrink involved" your answer is, "This is a reasonable solution, and it's good for all of us. Let's wait for her to come around."
The real issue is that SD25 is replicating victimhood because it's proven successful in getting rescue attention.
Let them work that stuff out.
You keep modeling reasonable behavior and good boundaries
