I'll assume the following from your post:
1. You are a U.S. citizen residing in Texas.
2. Your business would be registered in Texas.
3. The brothels are located in Mexico.
4. The services provided by the brothels are legal under Mexican law.
5. The brothels do not offer the services of prostitutes younger than 18 years of age.
If all five of the foregoing are true, you probably would not violate a Texas or federal criminal law by transporting customers to the brothels over the U.S.-Mexican border. I can say with relative certainty that you would not violate Texas state law because its reach stops at the state's borders. I'm no authority on international law, but I believe that a federal court would only have jurisdiction to prosecute you under a specific federal statute, and I know of no U.S. statute which makes facilitating prostitution by transporting customers over an international border a crime, assuming the conduct constituting prostitution in the foreign county is legal under the laws of the foreign country. However, there are federal laws that make transporting, coercing, or enticing a person to engage in prostitution which is
illegal in the foreign country a crime. See 18 U.S.C. §§ 2421-2428. Examples:
18 U.S.C. § 2421. Transportation generally
18 U.S.C. § 2422. Coercion and enticement
The problem, of course, is the potential that your business could be linked to child prostitution, which is defined under federal law as commercial sex with a person younger than 18 years of age. For example, note the following response to a FAQ on the U.S. Department of Justice web site:
Q. I am a travel agent. Do I have criminal liability if one of my clients goes overseas on a sex tourism trip?
A. It is a criminal activity to knowingly arrange, induce, procure or facilitate for profit the travel of a person when you know that the person is traveling for the purpose of engaging in illicit sexual conduct with minors.
See
U.S. Department of Justice, Child Exploitation and Obscenity Section, "Trafficking and Sex Tourism FAQs".
Personally, I would never even conceive of starting such a business because the risk of involvement in child prostitution is just too great. One only has to open a newspaper or turn on the TV news to learn that the Mexican cities on the border with the U.S. are in a state of anarchy because of the drug traffickers. In such an environment, or really in any environment, could you guarantee that the brothels to which you're transporting customers do not use underage prostitutes when you don't control their operations? If you're answer is anything less than an unequivocal "yes," you might think very, very, very seriously before starting such a business.