Yes, having a BPD step kid (no matter the age) can be overwhelming.
Step parenting is already pretty hard.
Your husband is probably enmeshed in the dysfunction and lacks the perspective to see how their dynamic harms both his D and himself, not to mention you.
My SO is a great guy in every other way, he tends to be unskilled when it comes to his D20. I've had to become the emotional leader.
Being an emotional leader means role modeling what it looks like to establish good boundaries. For example, if someone rages you can say, "I want to hear what you have to say. I will not be able to hear it when you shout or yell, so I will leave when that happens. We can discuss things next time and try again."
Or, you can say this to your H in advance and drive in separate cars. Most people with BPD have no boundaries so it falls to us to enact them (talking about them is not enough, in my experience). Codependent people tend to have weak boundaries. That leaves us

I slowly became skilled with uBPD D20 myself and SO started to notice changes. I took matters into my hands because the alternative, me trying to change him, only made me more frustrated and him more anxious. Maybe we can walk with you through different issues that drive you nuts and suggest things that might work. Context tends to matter, including what you are/are not prepared to do, and how much resentment you feel, and whether you want to coach or need to take care of yourself first. Because putting your needs first is probably job number one for any of the skills to be effective.
We are a work in progress. D20 arrives next week and I have thought through every boundary issue like I'm preparing to scale Everest.
