Hi RTSibling, so glad you reached out! We are here to help, and I'm so sorrry you are going through this with your sister and mother. It's not uncommon for pwBPD to become abusive. The fact that it's toward your aging mother is most disturbing. Is she able to care for herself and/or remove herself from danger?
Here is a good resource to help you get started on this path. Many blessings, please keep us posted.
What If I Suspect Abuse, Neglect, or Exploitation?
https://aoa.acl.gov/AoA_Programs/Elder_Rights/EA_Prevention/WhatToDo.aspx