I have a feeling that there could be more to this than any of us, including the OP know.
mosaicwarts, you mention that your son has mental health issues, which I imagine has caused you a lot of anguish. Please bear in mind that it's possible that your DiL might also have such problems.
I am only saying this because of something that happened in our own family some years ago, where one inlaw refused to get out of the car when 'attending' a family event. Eventually, that person agreed to enter the house, but all except the person whose big birthday was being celebrated (plus partner) had to remain in the garden or another room. At the time, it caused a huge amount of (largely unspoken) upset (especially to me, who had organised the celebration for one of my parents), but since then we have learned of the previously hidden/unacknowledged mental health issues that were behind it.
It's just possible that your DiL struggles with issues of her own and simply couldn't face you, for reasons that are beyond her or your control
Shall we reboot our cartoons thread again? 😁
Bought the wrong Hot Cross Buns 😩