Hi nevermytwin and welcome to the board. We have a lot of posters here with siblings with the disorder so you are definitely in the right place.
You did an excellent job with setting limits on the phone with your sister. Even better that she took it so well and was receptive to a conversation about her possibly having BPD.
I did go through out list of book reviews to find a hopeful book directed towards people with BPD. I came across this but I have not read it so I can't say whether I agree with the review or not.
https://bpdfamily.com/message_board/index.php?topic=68398.0I know you said you have read a lot, I assume about BPD

. Which books or articles were the most helpful? We have a lot of articles here as well but he best benefit of this site is to post your own threads and reply to others so you can build a support network. I find it very helpful to bounce things off the other people here. They always support me and definitely challenge me in the best ways. We get it here.
I hope to see more of you around and if I come across any more appropriate books I will post a link. Take care.