Hi Urmysunshine and a warm
You certainly have your hands full with pwBPD in your life -- that's common around here, so you're in good company.
How old is your sister and how old is your daughter? Does your daughter live at home?
Do you experience the same challenges with maintaining a relationship with your sister and your D, or are the challenges different with each of them?
If I had to pick only one skill for effectively maintaining relationships with pwBPD, I'd probably point you to
emotional validation.
True validation (not just sympathy, pity, blanket agreement, etc) builds connection, which sounds like is one of your goals.
When you have a minute, take a look at that workshop. Anything there stand out to you?
Glad you're here;
kells76