It happened too with relations of mine.
The man had three grown up children with his first long-time wife. She died before him and he remarried some years later for a relatively short duration of his second marriage (no children) until he died. The quarrel about the location of the grave was between the widow and the children of the dead man with his first wife.

That is a case in which I would intuitively side with the children and not with the second wife and first widow of him. My feeling who is right depends a bit on the duration of the marriage.

Wolfgang