
i'm pretty new here myself, but i'm not new to therapy, so I kinda get what's going on with you. Being with someone with BPD will severely mess with your head and make you wonder about all kinds of things you've never even THOUGHT of to think about!
Looks like you are hoping for some peace in the situation. I suppose that's where my mind has gone a lot once I realized what was probably going on with my husband of nearly 37 years--to that "help me understand! Help me make peace with the situation!" The hope for peace can only really come from what actions we ourselves can take. It can't really depend on more input from the BPD person and maybe not even from her family (who will maybe be more on her side no matter what.) It will be the work you can do with your therapist and the reading you do on this site and the questions you ask of people who have been dealing with this issue for years.
My own input would be that while it is common for pwBPD to have risk-taking behavior, it isn't always in the sexual realm. My husband has been in only 2 sexual relationships, one with his first wife and one with me. So that isn't necessarily a sign. And I've read that pwBPD have not always been abused as children, though my H was, so that's not always a for-sure either. And just because your ex loves her parents and is quite tied to them doesn't rule out any abuse, cuz we often do weird things trying to BE loved by our abusers.
I have been much like you in the "wanting to know things for sure" in the earlier days of exploring this, exploring some of the awful stuff that happened in my family of origin, but some of it there is no one alive to answer the questions. So I've had to work toward my own understanding and acceptance separate from those people. It's a journey alright, but it's a worthwhile one. And it takes time.
I don't know if that hits any of your real concerns, I just wanted to share that part of what I've learned so far.
Keep learning and I hope your therapist is as awesome as mine!
Elpis