Hi Seeking Saunders,
Welcome to the group

You are not alone in having a difficult relationship with a family member. I come at this from a slightly different angle, my Partner has an undiagnosed BPD ex-wife (uBPDxw). Everyone here has someone in their lives either diagnosed with BPD or exhibiting traits of BPD we "get it".
Unfortunately, none of us will ever really know if anything did or did not happen between your sister and dad.
I want to share information on the Karpman Triangle with you because I can see triangulation in your story. Just click on the link for more information ->
https://bpdfamily.com/message_board/index.php?topic=108440.0I also wanted to share a couple of books on BPD that you might find helpful...
Stop Walking on Eggshells: Taking Your Life Back When Someone You Care About Has Borderline Personality Disorder by Paul Mason MS, Randi Kreger
and
Overcoming Borderline Personality Disorder: A Family Guide for Healing and Change by Valerie Porr, M.A.
When I first found out about BPD I did a lot of reading and found it really helpful to get a good grounding in what BPD is and what might be behind some of the behaviors I saw with my partner's ex.
Besides the conflict between your sister and dad and the rage you mention, what other kinds of behaviors are you seeing?
Again Welcome I'm glad you jumped in and posted. I have found this site incredibly helpful in terms of support, information, ideas and sometimes just a place to vent. I know other members will be along soon.
Take Care,
Panda39