Hi Mink
and

It really is a significant challenge to determine if the high conflict personality in your life has BPD. Even professionals trained to do so, sometimes hesitate to make the diagnosis, and none of us here are professionals.
What we do all have here is a wealth of experience and much of it sounds
exactly like yours.
It is going to take some time and some investigating of what is going on before you can make informed calm decisions.
This site is a great place to do that. There is a lot of information all over this site so please let me point you to a link that will help you today.
BPD: What is it? How can I tell?If you don't mind me asking, how old are you and your sister? And I am also wondering if your family situation is such that you might find a therapist to talk to for a little while, as you sort this out?
Both reading and posting here was very helpful to me as I worked to learn better ways to communicate with partner. There are things, tools, that do help.
I think its very commendable that you want to organize the family to help them cope.

Let us know if the link was helpful.
babyducks