If my dad's life insurance beneficiary was his dad who ped away prior to my dad, who does the money go to?

My dad ped away a few months ago. We found out that the beneficiary to his life insurance policy was his father (my grandfather) who ped away 20 years ago. I am the only child and my dad did not have a spouse, so legally I am next of kin. However, my grandmother, who was the beneficiary to my grandfathers estate, now believes she has a claim to this money. Does she have any legal rights to the money? Is there any truth in this?