To me one of the most important tools is having boundaries, but having boundaries with someone with BPD is not easy, they are expert boundary busters and will use weapons like FOG (Fear, Oligation, Guilt) or emotional blackmail to get what they want. This is what often keeps us "Walking on Eggshells".
More on FOG... .
https://bpdfamily.com/message_board/index.php?topic=82926.0
As in the above quote boundaries are for us to protect us, to teach others how to treat us.
I see two places where you could work on boundaries, those you create around yourself (what you will or will not tolerate) and boundaries you create around your relationship with your husband (creating time to focus on and enjoy your relationship without his daughter and her issues creeping in)
Do you have some boundaries you might like to set? I also want to point out that you can only create boundaries for yourself... .one of the first lessons I learned coming here is that the only person I truly control is myself. You can't make your husband or his daughter do things they don't want to do, but what you can do is create boundaries and change what
you do, to make things better for yourself... .sometimes though, those changes can have ripple effects.
If you decide to set some boundaries don't be surprised if things get worse before they get better. I often share my simple analogy when talking about boundaries... .
A little kid asks mom for candy, mom says no... .kid pouts. Little kid asks mom again for some candy, mom still says no... .kid whines. Little kid asks mom again for some candy, mom again says no... .kid has a full on melt down screaming tantrum (extinction burst). What happens if mom gives in and gets the candy? That little kid has just learned that having a screaming tantrum will get them what they want. What happens if mom doesn't give in? The kid learns that no means no and he gives up... .this doesn't mean that the boundary won't be tested again the next trip to the store... .consistency is important here.
More on Boundaries... .
https://bpdfamily.com/message_board/index.php?topic=61684.0https://bpdfamily.com/message_board/index.php?topic=167368.0More on Extinction Bursts... .
https://bpdfamily.com/message_board/index.php?topic=85479.0I hope this information is helpful in your situation

I also encourage you to read the posts of others around the boards. I'm here because my Siginificant Other (SO) has an undiagnosed BPD ex-wife (uBPDxw) and I found the relationship boards helpful to get a feel for what his relationship with his ex might have been like and the children of a BPD Parent Board gave me an idea of what his daughters might feel and be going through.
For example you might want to visit the Son or Daughter with BPD Board
https://bpdfamily.com/message_board/index.php?board=4.0 for support and ideas in terms of your husband's daughter. That said this board should be a great home base for you... .there are many of us in step-parent roles and know that there are situations that are definitely specific to that role.
Take Care,
Panda39