Does my grandmother have to probate my grandfather's will if he is leaving everything to her?

Grandfather passed away in 1984. He left everything to my grandmother in his will. At the time of his passing he had 2 surviving daughters and 1 surviving son. Son was killed in a car accident in 1987. He had 2 young children. Oldest daughter passed in 2011. Grandmother's will left everything to the oldest granddaughter and the only surviving daughter. Grandmother's will was probated. Granddaughter bought out surviving daughter's half for ~$24,000. Surviving daughter says deceased son's children need to sign their rights away to the property so granddaughter is sole owner.