Hi Nomore4me,
Welcome to the BPD Family
You are on the right track, you're right can't change your sister (make her do, feel, think things she doesn't want to) but you can change how you interact with her and that can change the dynamic between you.
You can learn to understand BPD and what it is, and you can learn new tools and approaches to your sister. This will likely take time just as it does whenever we learn something new, and it might be uncomfortable for you and her as you learn healthier ways to do things. Sometimes it can be about getting comfortable with those uncomfortable feelings.
It is okay to make you and your comfort level with your sister a priority...this may take getting comfortable with the uncomfortable because it can be hard to put our own needs before the needs of others. You are not being selfish you are taking care of you and self care is really important when you have someone with BPD/BPD traits in your life.
You set a boundary when you refused to engage with your sister and boundaries in my opinion are one of the biggest tools in the toolbox. Boundaries are not about hurting or punishing your sister but are about protecting yourself from stress, verbal abuse, over the top demands etc.

Links to more on Boundaries...
https://bpdfamily.com/message_board/index.php?topic=61684.0https://bpdfamily.com/message_board/index.php?topic=167368.0I'm really glad you found us and decided to jump in, there is a lot of information, tools, ideas and support available to you here. Everyone here has someone in their lives with BPD/BPD traits so we all "get it". I was amazed when I first arrived and read the posts of other members how similar our stories can get.
Again Welcome!
Panda39