Ignorance of the law is usually not a defense. Explain the situation to the judge and see if he will give you a break. If not, pay the ticket and move on. It's not the worst thing in the world. And next time if you're not sure about the legality of game or fish, don't shoot or put it back. Good luck to you and happy fishing in the future.
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The alleged violation appears to be codified in the 31 Texas Administrative Code, 57.981 which you can look up here:
and, specifically here under "drum, black":
I don't normally deal with these kinds of cases, so take my conclusion with a grain of salt. However, the rules do appear, at least at first glance, to cover your situation by stating that "[o]ne black drum over 52 inches may be retained per day as part of the five-fish bag limit." I am assuming that both you and your brother in law have fishing licenses, so there's at least an argument to be made that you've not violated the regulations because you had no more than two over-sized black drum specimens between the two of you. Of course, my spur of the moment interpretation of the rule may not be accurate, in which case you might be stuck with the ticket. I'd talk to an administrative law attorney in your area, just to be sure.
My answers to the questions are for general informational purposes only and do not establish an attorney-client relationship. Accuracy of answers is not guaranteed, as my answers contain broad assumptions -- as such, you may not be able to rely on the answer under the facts of your specific case. There are no "one size fits all" legal solutions because even slight changes in fact situations may require a material variance in the applicable advice. Once again, I am not your attorney.
Sounds like the "old man" and game warden might be buddies and you got suckered. It happens. If you can't find the "old man" and get him to tell the truth I think you're going to have to pay the fine.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.