Based on my somewhat extensive (and painful) personal experience with these kinds of things, I strongly urge you to see an attorney, preferably a very good one. I might try to find a large law firm and ask the estate planning attorney to team up with a litigator and an oil and gas attorney, if there is much money at state. Also, if your mother's state of residence is different from the location of the minerals, it is possible that laws of two states are involved.
A good preliminary question for you and your siblings is whether your mother should straighten this out with her attorney without you being involved, so that nobody can possibly claim you pressured her. For example, you (and your siblings) can have your own attorney, and your attorney can talk to your mother's attorney, and you and your siblings can later truthfully state that you did not attempt to pressure your mother in the least and in fact didn't even discuss it with her. By having your step-father sign, she may very well have given him grounds to claim part ownership of the mineral interests and thus the royalty income. She needs to get this straightened out, the sooner the better.
If it were me, I might even chat with two attorneys, from separate firms, to see if they both have the same take on it. I have found some attorneys are a LOT better than others--another thing I found out the hard way. For example, some estate planning attorneys are trust-and-tax oriented and have no clue how things play out in an intra-family post-death litigation.
Good luck. Proceed carefully.