Welcome to PC.

I'm not sure what led to the ceased communication between all of you. It's beyond frustrating to watch helpless from the sidelines. In my own recovery from domestic abuse, the words of wisdom that stood out to me was that if there is at least one rational caregiver in the family, then the children stand a chance to overcome whatever it is within their homelife.
If your son is going through marital strife because of an illness within his home, would it be possible to let him know that as a silent observer you are there with open arms when the time comes that he seeks support? And to let your grandsons know you are available to them emotionally, spiritually and physically?

Are you connected with a counselor to speak with and help get through the grief and disappointment?