Hi sisLa. Glad you found your way here though I am sorry for the circumstances that brought you.
I wish I could say that I have heard of situations where a family was able to get a resistant uBPD to get help, but I can't or if it has happened I think it is very rare. It is hard to get someone who is mentally ill to accept that they have issues and then to get help and then to follow through with it. For therapy to work they have to accept that they have a problem and they have to want to get better.
The good news is that you, your husband and their father are willing to learn better ways to interact and communicate. There are plenty of articles here on that and if you look at the very top of this board, you will see a post titled Lessons
https://bpdfamily.com/message_board/index.php?topic=108307.0 and within that list there are articles that can help. Scroll down to the second post and there are links to specific articles that are aimed at better communication strategies and setting boundaries. Right now, the best way for you to help her is to establish boundaries that will help and protect you and stop any enabling behaviors (if there are any).
As for financial support through something like SSDI, there usually has to be documentation including a diagnosis and they will have to see effort at getting treatment. (I just finished the process for physical impairments but I know the same is true for mental/emotional disabilities). Have you checked out resources like SNAP (aka food stamps) and medicaid? Contact your local community action center and or take a trip there and see what they have to offer her. She may even qualify for SSI though i do not know of the requirements for that.
I do wish you the very best of luck with this. I hope I did not come off as all gloom and doom either. There is hope, but that hope is for making things easier on you,your husband and father in law first and foremost.