This is sad, but I am not sure you can do anything to change the situation.
You say you and your son have had a difficult relationship since his father died. Your DIL can't be expected to think that this is her husband's fault at all, or even in part. And if, as so often happens these days, she is not close to her own parents, she may have no desire to get to know you, or see the necessity for grandparents in their child's life.
Stop inviting them and near Christmas send a present for the child and a card to the parents. Most people are willing to accept gifts for their children , and a present to the child and a card to them signalss' that you still want to keep in touch.
All youngsters are so busy these days and most only keep up with friends and family via social media. To our generation this is the same as not keeping in touch, but not to them.
If it makes a difference that you are less wealthy than they are, and it might well, if either your son or DIL are snobbish, or like so many in their generation primarily concerned with monetary wealth as conferring status, then honestly you are better off not seeing them.
None of this will make you feel better, I fear, but by not being seen to be "pushing" or "nagging" them, if this is what they feel you are doing, there is a slim possibility that they might come round to inviting you.
Do you smoke? Most young parents won't let their babies anywhere near smokers or their homes now, and some feel the same way about a home that has pets.
If it is any help, there are tons of other grandparents facing the same dilemma.
It is not necessarily your fault at all. Most young people today are happy to live their own lives without bothering about family. Again no consolation, except that again, this has nothing to do with anything you may have done, or said.