This is my own experience with LC/NC and a family member. In some cases, it is necessary- and I don't judge anyone for doing it if it is the right thing for them. In my own experience though- it's not a solution when that family member is also connected closely to other family members and you still want contact with the other family- because the person you wish to avoid will be a part of that unit.
It also is abrupt, and sometimes this can cause a reaction from the pwBPD, who can also solicit other family members to "their side". Rarely in a family is the issue with just one person. Other members may be rescuers/enablers.
My only and short failed consideration of NC with my BPD mother was in college when a counselor suggested it- for good reason- issues were making it hard for me to focus on my studies and affecting me academically and emotionally. BPD was not really well known at the time. I'm sure that hearing about my mother's behavior was significant enough for the counselor to suggest it.
I tried but still wanted contact with my father, and they were a pair If I wanted contact with him it would have to include her.
It was many years later that I learned about BPD and the family dynamics. In your situation, going NC with your sister is likely to include other family members acting as rescuers, including possibly your parents. You don't want to cut contact with your entire family.
LC for me was not just less frequency of contact but the content of the contact. It meant not sharing personal or emotional information, avoiding circular and emotional discussions. I think a good approach is to not make a statement or anouncement, but a "slow fade" to both frequency and content of communications. She might call, and you can be busy. Or she starts with emotional content or accusation and you say "I need to go, someone is at the door, or you have an appointment, or whatever non emotional reason to get off the phone. You don't call as often or not at all.
One of my guides for content is- if it were on the evening news, would you care? So I might say "child's soccer team won today" "I saw this great movie" rather than to have a personal emotional topic. If she asks personal info- then say "I don't know or it's not important" or some deflection, or get off the phone. There still is contact, but the content is less emotional, less reactive, less drama.


