@Lunita, Welcome.
It's great that you're seeing a therapist who will support YOU, and also great that you're actively thinking about how to prepare and get the most out of this time.
Some things to consider. If I had to guess, you might spend the first few sessions providing all the context you've shared here - explaining all about your H, D, SS, H's ex and parents, your ex and parents... i.e., everyone else.
What about you?
My suggestion is to use the time with your T to connect with yourself. Try not to worry about everyone else.
Of course, all these relationships are a big part of who you are and have an important role in how you're feeling - especially your H and D. That's fair, and understood.
But I'm sure you're aware that people rarely change. This may be extra-true for BPD people who seem to have antibodies that are especially resistant to any form of externally-motivated change... A hard truth is: individuals with BPD need to find their own path. There are some things you can explore and practice to reduce triggers and conflict, and there are good resources for these exercises on this site. But that generally only goes so far.
In my journey, when I considered "radical acceptance" I initially understood it to mean that I would have to accept my pwBPD's behavior unconditionally. However, I eventually came to understand that the true meaning of radical acceptance, at least for me, was coming to terms with things about myself - that I could or could not change. This was an important breakthrough. And so I encourage you - and everyone else dealing with BPD relationships - to look inward as much as possible, as part of the individual process.
https://bpdfamily.com/message_board/index.php?topic=89910.0While you're exploring - Consider reading up on "attachment styles"
https://bpdfamily.com/message_board/index.php?topic=279028.0and codependency. You may find some useful concepts...
https://www.bpdfamily.com/content/codependency-codependent-relationshipsWishing you some peace this weekend, and as you go forward.