Sorry for your loss. As for the two of their sons do not want to attend their father's funeral. It sounds to me like this is a lame excuse.

There is something else going on that you do not know about. Who knows what it is.

This is more of a statement against your grandma than it is about the grandpa.

We all know funerals are for the living.

I suspect you should reach out to the two sons anyway. They are having to deal with the loss of their father.

Best wishes