Hi Chiara,
I'm glad you've found our group, welcome!
I come at BPD from a slightly different angle, my Partner's ex-wife is the person in my life with BPD/BPD traits she has not been diagnosed but the shoe fits. It isn't really about a name anyway, it's about the behaviors and how hard they can be to deal with. It sounds like you are in it at the moment too. It's great that you have a Therapist's support and now you have us too.

What does the situation with your sister look like? Is it just you and her? Do you have other siblings? Do you live near each other? Have you done any reading on BPD, there are a lot of good books out there. What do you find most challenging when it comes to your relationship with your sister? We can likely direct you to some tools and information if you point us in a direction. It's all about supporting each other around here.
I hope to hear more of your story when you are ready to share it.
Take Care,
Panda39