OK, this is what I thought was the situation. I want to emphasize that nothing I state on this Forum constitutes legal advice or the practive of law, it is simply my personal opinion based upon my experience. So your Dad owned 100% of the minerals/mineral rights BEFORE marrying your stepmother, that means that the minerals were HIS "Separate Property," not THEIR "Community Property." Then your Dad died without a Will, or at least without one that went to Probate Court.
So pursuant to the laws of descent and distribution in Texas, that means your stepmother would be entitled to a "Life Estate" in one-third (1/3) of the minerals/mineral rights, meaning that she would have that 1/3 for her lifetime and could NOT sell them, and the remaining two-thirds (2/3) of the minerals/mineral rights would transfer in fee simple to all of your Dad's children, divided equally. "Fee simple" just means that the kids COULD sell their portions at any time, unlike your stepmother's Life Estate portion. So if your Dad had four kids, each kid would be entitled to 2/3 divided by 4, which is 2/12, or 1/6. Then when the stepmother died, HER 1/3 would AUTOMATICALLY get divided up among all of your Dad's children also, REGARDLESS OF WHAT HER WILL MIGHT SAY, so again, if your Dad had four kids, each kid would then own 1/6 + (1/4 * 1/3), or 1/6 + 1/12, or 3/12, or 1/4. Therefore, from the perspective of these minerals/mineral rights ONLY, the wording of your stepmother's Will or the status of her Estate is IRRELEVANT! Once she died, all of your Dad's kids then owned 100% of the minerals/mineral rights, divided equally. If they had sold their portions after Dad died but before stepmother died, obviously that would be different. Also, if one child predeceased your Dad the ownership would be different too. But basically, the law is set up so that when a person dies intestate, his or her separate property eventually ends up with his or her blood relatives (adopted children are treated as biological children).
NOW, here's the thing. You say that your Dad has been deceased for twenty-three years, and your stepmother has been deceased for three years, right? I need to know the answer to two questions, and they are extremely important questions because your answers to them could dramatically change what I wrote above and alter who was/is entitled to the benefits arising from these minerals/mineral rights over the last twenty-three years, benefits such as bonus, royalties, shut-in payments, delay rentals, etc. Here are the questions: "Exactly WHEN was the most current Oil and Gas Lease signed, and exactly WHO signed it?" and "Since the time when the most current Oil and Gas Lease was signed, did any one or more of your Dad's kids sign a Lease as to these exact minerals or a Ratification of such a Lease?" Please respond when you know the answers.