Welcome shandyblerg:I'm sorry about the situation with your sister. I can understand how it makes you angry and frustrated. I have a uBPD sister that I'm currently not in contact with (NC).
. . .another day consisting of my sister abusing my mother verbally and emotionally and then splitting-ly turning to me for pity about the perpetual perceived sorrows that are her life... .I feel guilty for how angry I am with her but at the same time, I don't. She literally makes me want to get so far away from her... .
Are you able to share more details about the circumstances around your sister's abusive behavior with your mom? Is your sister currently getting any treatment for BPD (meds and/or therapy)? What are her "perceived sorrows" about her life?
It is so much easier to give advice and share tools with others, then it can be to have to employ and use the tools in our own life. We are all human and regardless of our background and training, we all have times when we are driven to anger and struggle with situations. So much in regard to BPD seems to defy logic and understanding.
What did you say to your sister when she turned to you for validation? Is your sister abusive to everyone, or mostly your mother?