Hi
Witsend!
I'm glad you have joined us! Welcome!
From what you have described, it sounds as if you are suddenly putting things together as far as your sister's behavior, and you are connecting the dots. You've suspected things were not right and yet you managed to make it through all these years.

How did you first learn about BPD? My mom was an uBPD, and she never gave up hoping I'd move close to her to live, like my brother and sister did. I always figured they were stronger than me or didn't mind her behavior, but I knew I could never do it. She'd control me like she always did. I can understand your reluctance to move into the same house with her, especially now that you've seen how she lives.
She is so old that I cannot see cutting off relations with her. I know she is ill. I have great compassion for her, but I don't want to die in the process of trying to save her. It is an untenable situation.
I think it's great that you know you cannot save her. That's an awesome big step.

Do you think it is possible for you to still remain in touch with her but not give her money or do anything to save her? What would that look like to you? Are there organizations that can step in and help her?
My DH took a class at our local rescue mission, and I still remember one of the most important things he learned from it: "You can only help someone as much as they want to help themselves." In other words, if a person really doesn't want help to get out of the place where they are, no amount of work and help (financial or otherwise) will help them to better themselves.
I am super glad you've made an appointment to see a T. That can provide lots of support for you. Are there any books you've read about BPD? We have a great book list if you'd like some suggestions.
Wools