I lost my DH in April, and do understand that family fall-outs just add to the pain.
I'm "fortunate" that the only ones who treated me badly were my Stepchildren and my brother - well good riddance to both of them.🙄
But, it's obviously different with a son and GC.
Your DIL sounds like a stroppy little madam, but, your son is the one really at fault here.
There is absolutely nothing to stop him visiting you, with your GCs. Your DIL doesn't have to visit.
If he cannot/doesn't stand up to her, then I'm not sure there's too much you can do.
I really wish people would understand spousal bereavement - it's horrendous, and it costs people very little to give support.🙁
If possible, I'd enjoy the relationship you have with your daughter, grieve for your husband, and gradually try to rebuild your life - with or without your son and GC in it.
Condolences on your loss x💐