I want to commend you for being frank and open. It's hard to balance that sense of "well, we should be helpful, we should take her in" with the core anxiety of "I don't think my family can handle this".
Am I tracking with you that your husband was with a pwBPD (person with BPD), with a then-6 year old daughter and then-14 year old son, neither of whom were your H's bio kids? So now, the 6 year old is 14, so the 14 year old is 22 and not a part of this current situation?
So the kids involved are H's "stepdaughter" age 14 now, and your bio child age 16. Is your 16 year old's dad involved in his/her life?
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It makes sense what ForeverDad mentioned... there's a lot of unshared information right now. It seems unlikely that a placement could be made without knowing more about what other family members in the household are like, so I'm with you, I'd expect some kind of reaching out from social services, if they're serious.
It's good that you've been upfront with your husband about what you plan to say when you're contacted. I'm sure there's been enough ambiguity in your lives already. I hope it went OK to be honest and straightforward like that.
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My H's kids' mom has many BPD traits. Not getting the full picture comes with the territory

I usually function better when I have more information, so while in the past I'd feel like "I'm just the stepmom, I can't do anything", more recently I've been more proactive about getting info about the kids -- calling the dentist, calling the school attendance office, etc. When I have more info, I can make better, less anxious decisions.
I wonder how it'd go for you if you just started calling social services, described what was going on, and asked some really specific questions. For example, the question of -- even if SD14 (14 year old stepdaugher) were to be placed with us, what does that mean legally? Does that mean her mom has had her parental rights terminated? Or is it more like "foster family"? Or what?
Another idea is that a lot of lawyers will do a free initial consultation. Typically it's around 30 minutes but ours was nice and it was like an hour. You can come up with a bunch of questions beforehand and get a sense of how courts work in your situation, if there are any legal details that aren't obvious that you need to know about, etc. Getting that kind of info could go a long way to helping your family make a healthy decision. You can also post on avvo.com, it's free (but very brief) legal advice. Check online beforehand for local lawyers that specialize in juvenile, family, and/or adoption type fields -- something related to "child placement" or "custody law" or something.
Really glad you found the group, and thank you for just being open and laying it all out there. That takes some courage.
-kells76