Welcome Qtip: Sorry you are having problems with your sister. I have a uBPD sister myself. She appears to fall within the high-functioning BPD label.
You can't change her, but you can make things better for you, by using certain strategy and communication skills. Have you read any book on BPD? I found the book, "Walking on Eggshells" helpful for me.
The links below lead to some specific information that will likely be helpful:SETTING BOUNDARIES COMMUNICATIONS OVERIVEW VALIDATIONAre you still seeing a therapist? Do you have to interact with her very often? Do you live close to her?