Hello and welcome! I am glad you found us and posted. We have several members here coping with a sibling with BPD and all of us are dealing with a family member with it. Several of us deal with depression and anxiety and also have some BPD traits so you are not alone in that either.
We can support you as you determine what kind of relationship you want with your sister and how to navigate you way to it. It is very helpful to post not just in our own threads but also in others as it helps us to see our own situations and sometimes we can see solutions we otherwise would miss.
Does anyone else feel like they lose their best friend when they are in crisis? Do I need to not be so close to her to protect myself? Purposely make friends with someone else?
Well, i think it is possible to get to a place where you can manage your expectations, practice radical acceptance and practice good boundaries and self care. It is a very good idea to develop a support network of people who care and who you can have fun with and enjoy being with. We can serve as par of your support system but it is also good to develop one in real life as well in addition to your relationship with your sister. It does not have to be an either or situation.
Again, I want to say welcome. I am glad you found us.