Since you dad died without a will, and given that he ownded the mineral rights before being married, upon his death one half of those mineral rights should have gone to his wife and one half should have gone to his children. Then when your step-mom died her 1/2 share would go to whomever is named in her will. If she had no will then her share would go to her children, if she has any, and if not then to other relatives of hers such as her parents or siblings.
As of now your best bet is to engage an attorney to sort this out - first someone (you?) will have to be named as executor of your dad's will, and someone else executor of your step-mom's will, which would then allow retitling of the property into the names of the proper heirs.
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