lostatsea,

. I am sorry that you are feeling so angry about you sister's behaviors. I can certainly understand how frustrated you must feel after she has accepted help and turns mean again. You will find that many here understand what you are feeling, and find much support. I know this site has certainly helped me immensely.
No apologies needed for being negative. We are here to listen!
I suggest to read all that you can about BPD. We have a very informative site. You will find much information from the home page there are articles, videos with the article, resource links.
First here is a link describing BPD:
Video-What is Borderline Personality Disorder?Another one is about conflict. You can substitute your sister for the word "partner".
Video-Tools to Reduce Conflict with a person suffering from BPD When a family member has BPD, the illness can negatively everyone in the family system, including children, siblings, and in-laws. Senior members on the [L5] Coping and Healing from a BPD Parent, Sibling, or Inlaw board are experienced with and can help you with setting boundaries, finding relief from FOG, encouraging self-care, improving your handling of relationships impacted by your BPD relative, and pursuing a path of recovery from traumatic experiences. The validation, information, and support will give you strength on your journey.
Link for relative board:
[L5] Coping and Healing from a BPD Parent, Sibling, or Inlaw BoardLet me know if there is anything else you need. You can post on the Board link when you are ready.
I wish you peace.
peaceplease