Hi and

Can you tell us a bit more about your sister and some of what she struggles with and what you already do to support her?
In general, I would recommend learning about the behaviors associated with the disorder and tools that will help you cope and support her. How's that for a non-specific answer?

I don't mean to be vague it is just a broad question to try to answer. How is your relationship with her now? Has she been diagnosed for long? How old is she?
There is a book titled
"Loving Someone With Borderline Personality Disorder" by Shari Manning that i find quite helpful in terms of understanding some of the behaviors we see and learning productive and supportive ways to support our loved one and take care of us as well. I specifically like the book because it helps us have a more productive and constructive relationship with our pwBPD (person with BPD) rather than one full of confrontation and one that takes a more adversarial approach that a lot of other books take.
A big part of being in a relationship with a pwBPD involves having good self-care, strong boundaries and compassion for both us and them. We can support you as you navigate your way to your goal.
What do you struggle with in terms of supporting her?