If a deceased person's last will and testament is not upheld to their wishes, how can this be rectified?

My great grandmother left $5000 CDs to her grandchildren to recieve on our 18th birthdays. However, my father and aunts took the money, and me, my brothers, and cousins have never recieved any of it. My great grandmother had amazing relationships with us children and hated her daughters kids (our parents). So it really has bothered us for many years that they did this not only to her but to their own children. Especially when my father never chose to be apart of our lives. So my question is who would be the best person to look into the will and see what happened to the money?