It depends on what your great grandparents and grandmother’s Will or probates state. You can get a copy from the court clerk in the county where their estates are probated or possibly in the county were the minerals are. If the Wills provide for who is to inherite minerals upon your grandmothers death, then that is who will inherite even if your grandmother died without a will. If your great grandparents will does not provide for who is to inherite upon your Grandmother death, then her estate will be distributed according to her will, If she doesn’t have one then her estate will be distributed as provided for by state law which would automatically provide your dad with an interest, the only way your stepmother would get an interest is if your grandmother provided that she should in her will or if your dad got his inheritance and either deeded it to her or died and left her his interest in his will (which you would have the right to protest in court if written while he was ill). If your great grandparents or grandmother’s will does not provide who is to get the interest upon your father’s death, and your father died without a will, your stepmother would get an equal portion with all his natural and adopted children but not all the interest. Hope this helps. Sounds like she is trying to mislead you.