Do you mean burglary with a firearm or theft of a firearm? It depends if the possible charges are state or federal. You should have your brother consult with and retain an attorney.
It is five years for burglary or theft. Furthermore, if the charges were already filed within the five years but they did not bring him to court then there is a very good defense that Pa. Rule of Criminal Procedure 600 has been violated and the charges should be dismissed based on that. Just because he is in jail doesn't mean the Commonwealth gets to stop the clock and there is appellate court law on this point.
He needs to get a good lawyer that has handled these matters before.