Yummy, I am suffering in the same way as you. I cannot think of anything more I can do to make contact with my daughter, as the three children who still live with her have all blocked me on Facebook, with no explanation. I did send her eldest son a long pm pointing out that his mother's accusations were completely untrue and asking him to simply look at the careful accounts I left with them. No reply. The eldest girl, who lives with her fiance and their two little girls, won't get involved or tell me what her siblings think I have done wrong.
I have had to accept that I may have lost my daughter forever, to her drug addiction, but I live in hope that someday her children will realise that I love them and will always be ready to help them.
It looks as if the three of them will stay away from their cousin's wedding, which is really sad because he has always got on well with them. They must be afraid of offending their mother, who is supporting them all financially.
I don't feel that things are quite as bleak in your family's case. You are dealing with fairly normal resentments and jealousy, not mental illness. I did think that it was wrong of your DS to comment on his nieces's toilet training and I think if I were his SIL I might have replied angrily too. I don't think telling them you are broken hearted will do any good - they are all too certain that they are in the right to be sympathetic. You would probably be told, as I have before , not to feel sorry for yourself. Have you made it clear to DS1 that his comment and his subsequent aggressive behaviour towards his SIL were completely out of line? I get the impression that your DIL is being blamed for all of this, and whilst it may be correct that she has caused trouble in the past, and none of you like her, in this case I do feel she has some just cause to be offended. Anyway, it is your son's job to deal with his wife, not yours.
Hard though it is to be deprived of contact with your grandchildren, I feel that 'sit tight and say nothing' might be your best plan. I fear that any overture you make might be misinterpreted. If you think it could work, you could write a short letter to both couples, just saying you can understand why they are upset, but that it is not your quarrel and you don't want it to affect their children. Good luck.