Aren't there grandparents laws that give them the right to see their grandchildren if the parents are no longer together? Whether or not Stern had any role in ANS and her son's deaths, he comes across as guilty, vindictive, sleazy, and sadistic. It sure made him look bad and stupid not to let the grandmother see her granddaughter.
Grandparents might get rights to visitation if they had acted in the place of a parent - say the child lived with the grandparents for some period of time - and that would fall under "the best interest of the child."
I'm sure there's exceptions, but that's generally the way it works.