HI sidra: I'd like to join
MUTT in welcoming you. I'm so sorry about the situation with your sister. I have a uBPD sister myself. My parent recently passed and I'm currently not in contact with her, except through her lawyer. My sister has had her issues through the years, but she exhibited classical BPD symptoms, when our parent's health began to fail and we had to work together. She painted me black. I was the one who went to therapy to try to make sense of what was going on.
How old is your sister? Does she have a job, or has she ever lived on her own? Is she currently in therapy or taking meds that can help with BPD?
Life can be very miserable for the person painted black (if that might be you). Is it possible that you can live somewhere close to your dad, but maintain a separate household. You could perhaps help him as needed and then escape to a safe and sane environment.
Are you able to talk frankly with your dad about your sister's condition? How old is your dad?
There is a lot of helpful information on this website. If you look to the right of the page, you will see several links to lessons.
For starters, some reading on
FOG and
BOUNDARIES could be helpful.
There is a lot of information her, so don't get overwhelmed. There are communication tools that can be helpful with people with BPD (pwBPD). Boundaries are for your safety and benefit and are something that will likely be important.
Have you read any books about BPD?