2422b means enticement of a minor to engage in prostitution. It is a very serious offense. There is a 10-year mandatory minimum sentence. http://www.law.cornell.edu/uscode/text/18/2422 Although the victim must be under age 18, the statute does not have an age limit for the offender, so a minor could be prosecuted under this law.
You did not explain if your brother was arrested, or if this is something your family became aware of and is trying to decide what to do.
He could be prosecuted in Federal Court by the US Attorney's office, or be referred to the local court's juvenile division. Even if the federal authorities prosecuted your brother, they would likely turn him over to the local juvenile probation authorities after the court proceedings are concluded, because the US Dept. of Justice and US Dept. of Probation are not really equipped to deal with Juvenile cases.
Your brother needs an experienced attorney. Your parents should either hire an experienced, local federal criminal defense lawyer, or request a public defender when your brother is brought to court.
It is hard to imagine how a 12 year old could violate 2422b. But in any event, in order for the feds to take the case, the Att Gen. has to certify that the case involves important federal interests and the crime is a crime of violence (it is). If the feds keep the case, it goes to federal juvenile court. A 12 year old is too young to have the case transferred to adult court. A juvenile proceeding in federal court takes place before the same judges and in the same courtrooms as adult court, but the procedures are different and the courtroom is sealed from the public. If there is a trial, it is held before the judge without a jury. This is a very serious offense and your brother needs a criminal defense attorney who is familiar with federal cases.
Mr. Richman is correct. If what you are telling us is accurate, your brother will not be tried as an adult. If the government wants to move forward, the Attorney General has to certify the case, as (correctly) noted by Mr. Richman. If the Attorney General does not certify the case, your brother will be surrendered to state custody. If the Attorney General does certify the case, your brother could be tried in federal juvenile court, where he would likely be tried before a federal district judge.
I began my career nearly 30 years ago as a indigent defender in juvenile court and I have extensive experience in federal court as well. If you are interested in having our office represent your brother, please give us a call.