What is a surviving parent's rights to a deceased child's estate?

My granddaughter lost her life to cancer @ 8 years old. (The parents separated and later divorced during this.) At the time the child had a trust from a lawsuit for complications at her birth. Both parents were awarded money and my granddaughter was awarded money when she was about 2 years old. Her father died of an accidental overdose & my daughter has remarried. My granddaughter's father's father is executor of his son's estate. He is trying to gain control over his son's (estate) and the child's mother's part of the estate & be in control of a trust for the child's younger brother. I know it is a terrible awful thing to do to my daughter but, is it legal?