My long term DP has a borderline sister and she has made her family's (and mine to a lesser degree) lives hell from time to time. She seems to be getting better and her folks get all excited and think they have her 'back' and then she goes down again. So hard. I really feel for you.
Don't know if your sister does this, but she has fabricated some awful lies about dp's whole family and, although she shows remorse and builds bridges when 'well', when she is down again she reverts to the 'other' side of her personality.
If I was you, I would offer arms length support - you need to protect yourself (and your own family if you have one) as her behaviour could take any turn. And I would not blame your parents - they have been through hell with her already, I imagine.
I don't say this lightly and have offered much support in our situation, but also work in this type of field and have a family member who is very clued up on PD professionally.
Please, any BPD people reading this, don't take offence. I just know how often our family relations have been hanging by a thread and indulging someone who exhibits these behaviours is a bit like feeding a fire.
There is good evidence that talking therapies such as Analytical Behavioural Theray and CBT can give good results. This depends on willingness to engage, of course.
Good luck and I have my fingers crossed for you whateverxx
