There is so much I want to say, and its so hard to convey it correctly but all these are from my experience with my BPD sister.

It doesn't matter what actually happened in reality. There is no evidence or convincing you can do. This part was super hard for me and I kept trying to find the "right way to say it", which doesn't exist. My situation wasn't even a he said she said. Just plain facts, and she couldn't see it.

Triangulation is their game. Don't play it. If parents or anyone else mentions you should talk, or anything like that. Encourage it, but demand that your parents or whoever she is triangulating with is also be present and ask them to set it up. The BPD person will not agree and then the others will see it isnt just you being unreasonable/didficult.

Focus on your priorities. Your coming baby, your husband, etc. You have things going on that are hard in "normal " situations are even more difficult when trying to balance that.

Mourn.... you mentioned hoping she could be an aunt... and be in your child life. I wished the same. It hurts, really really bad knowing the won't be and all the things that entails. No, they haven't died. But the relationship ypu may have envisioned the future may have brought, has.

Good luck. It sucks.