Hi Monkeymagic
Dealing with a BPD sibling can be quite tough. Has your sister been officially diagnosed with BPD? Has she perhaps ever received any kind of treatment or therapy for her issues?
I am sorry to hear that your mother is ill. I can understand why this would cause you to be extra concerned about the stress your sister causes. As far as communicating with your sister, we have several communication techniques on this site that you might find helpful. I've included some links:
Communication Skills - ValidationS.E.T. - Support, Empathy and TruthWe also have some material here about the things we can do ourselves to end the cycle of conflict:
Ending the cycle of conflictTake care and I encourage you to keep posting here