Spindle!

This is such a success story. I hope it brings hope to others who find themselves in a situation similar to yours.
She got on the plane! Five amazing words! I think that is also a testament to you and H, who must have used a lot of skills effectively to make this happen. Congratulations to both you and H.

She kept saying that it was her last time in NY and that she probably won't be allowed back. I know that hurt my husband's feelings because he has worked so hard to show her that he is trustworthy, and she still doesn't believe that she will be allowed to visit.
Those are hooks to draw drama. She will stop saying it after her first visit (if it were my mom, I would insist on a return air ticket), whenever that happens (distant future?). Right now, it's maybe just something she has to say because of the BPD?
Husband is having a tough time, and is really worried about her. He doesn't want her to end up in an abusive situation, and despite our encouragement to tell his sister that she wants them to move out, she seems set on moving into her house, but letting them decide if they want to stay.
Letting
them decide sounds like the BPD disorder again, always needing another crisis, or creating one if there isn't one on the horizon. It's not logical or rational to let
them decide. It sounds purely emotional. Having said that, it's her property, and she's an adult, so it's her decision. So if she makes that decision, it's not H's duty to worry about her in the abusive situation because she will have
chosen that situation, because
she didn't ask them to leave. If drama and chaos happens there, she will try to blame others, probably H, but hopefully he is OK with accepting that situation will have been
her choice. He does not have to accept FOG. The logical time to ask them to leave is now, with her moving back. Asking them to leave 6 months from now or a year from now would involve far more drama than at this time, but ultimately, that too is her choice. It's so hard to let BPD's learn to live with their own consequences right?
One step at a time. She got on the plane. That was a HUGE step. Great job for setting up the environment where she felt safe enough to see that through.
I am so happy for you and H. I hope he recovers from this change as quickly as possible. It's also impressive that he saw the whole plan through to the end. I can imagine he's going to be conflicted for a while. I wish the two of you joy as you move on with your own life together.