We're glad you are here, too! Welcome.
Having a family member with BPD can be difficult and confusing. You are right that your partner's sister may never see a need to work on changing her own behavior. But you and your SO do have the power to work on remaining emotionally detached from the sister's chaos and maintaining healthy boundaries. This may be more challenging for your SO since s/he will likely have grown up with Fear, Obligation, and Guilt. Has either of you ever considered talking with a therapist? There are also some great books on the market.
Here at bpdfamily.com there are lots of articles and workshops that can help you continue your research into understanding ways to cope with this disorder. One that gave me a

moment was Radical Acceptance for Family Members. The one on Values&Boundaries is also very good. I hope you will also keep posting on the message board. You are not alone in this!
Wishing you peace,
PF