Welcome Aguakaty: I'm so sorry about the situation with your DIL and son. There have been several other members who have posted recently with the same problem.
Unfortunately, we can't change people with BPD (pwBPD), only the way we interact and react. Is your son completely unmindful of your DIL's behavior? Might your son have an issue with being codependent?
Have things been difficult with DIL from the start, or was there an event (s) that you can identify as a turning point in her behavior? You might want to read about
SPLITTING. Sounds like your DIL may have painted you black.
They refused to get counseling with us and so we decided to go ourselves. after our third session with the counselor he suggested the book "stop walking on eggshells". We are shocked to see so many traits of BPD in our dil.
I went to therapy to explore what was going on with my sister. My therapist suggested I get the book, "Stop Waling on Eddshells" as well. There is a lot of good information on this website and you will find some links to the upper right of this post.
VALIDATION is another tool that can be helpful.
Learning various communication skills can be helpful. You will probably find that you will never be able to discuss anything controversial with your son and DIL (or have an opinion about things in their life). Perhaps, with some strategy, you can get back in their lives. You may have to just go with the flow, and accept what is possible (without debate and any conflict viewed from their side). You may not have a normal relationship, where you can discuss things and sometimes disagree.