What happens when the beneficiary and executor die before estate is filed?

My stepfather died . He had a will that named my mother beneficiary and executor if she outlived him. If she died with him then everything went to his daughters. His oldest daughter is also deceased but she was named backup executor should my mom not be able to do . If my stepdad & mother died together then his assets were to be divided between his 2 daughters. however, when he died my mother never did anything concerning his estate. The house was in is name only. can we petition to have his other daughter named as executor or does the original executor's children (deceased's grandchildren) come into play?