Hi jabar,
Iam sorry that you feel that you have lost your son and gc in this situation. Maybe there is a possibility that your DIL is threatening and manipulating your once loving son. The lies are there to divide and conquer and control. Your DIL feels threatened by the once loving r/s you once had with your son and she has to put an end to it to otherwise she probably feels that she will lose number 1 position in his life. Pwbpd can even be jealous of their own children so the r/s she saw between you all is something that isnt admired it is something that she wants soley for herself. Remember also pwbpd are all consuming and have a irrational fear of abandonment in any form so they will often make up lies to ensure that it doesnt happen.
I think that your DIL is probably enjoying watching you get all worked up and the more you retaliate the more she will do her worse. The last thing I heard was that my udd was saying that Iam not allowed to see my gc because I ruined her childhood with my (supposed) mood swings and abuse and that she was worried I would do the same to gc!!!
My question was to the s/w atm was...Yet it was ok for me to look after my gc in my own home, ALONE for years until it didnt become ok???

He coudnt figure it out either.

The best thing I think you can do is to save your time and money and put all of this behind you.
You know the truth and your husband knows the truth about the things she is saying....Maybe even deep down your son knows the truth and thats all that matters.