Hi there onesister
I'm glad you found this site. It's packed with a lot of information that's been really helpful for me in my own journey, and it's a very warm and welcoming community of people as well. I think you will feel well here.
Here's a link to some resources specifically for siblings or kids of people with BPD:
https://bpdfamily.com/portfolio-parent. You'll find a lot of useful articles and links there.
How come your therapist is advising you to limit your contact with your sister? What kind of effect is it having on you personally? Can you tell us more about the family dynamic in general? Is your sister younger or older than you? Do you live in the same household? How much contact do you normally have?
Looking forward to learning more with and from and about you