IANAL and I can not authoritatively answer the legal questions in the OP. Your idiot brother may want to visit a lawyer.
How much jail time if convicted? That is a question that can be answered only by your state’s laws and a judge. The laws may specify minimum and maximum penalties, but in the end it’s up to the judge.
Is your mom complicit if she doesn’t report him? I don’t know. I do know that if your mom backs out and says he had permission to use the card, then she will be liable for the charges. Not familiar with the situation as Zsofia describes it. If she just allows it to go as an unsolved crime and does not tell the authorities that your brother made the purchases, I see two problems, but a lawyer would need to comment for this information to be of value. First, she would be withholding information or giving false informationt to the police, which is probably some sort of crime. Just as bad, there are all these purchases that your brother still has, and she’s just letting it ride while the credit card association/bank/merchants have to cover the theft. That’s fraud. I suppose she could be an accessory after the fact, or part of conspiracy to defraud.
This may indeed be the case, though it depends on how the brother made the purchases. In person? Phone? Web? Normally, a merchant is required to check only the signature on the charge slip against that on the back of the card, and then can lose nothing if the transaction was fraudulent. The contract with the associations generally says that. If, however, the card has your mother’s name, and your brother either signed his own name or a poor forgery of your mother’s, the bank may not reimburse the merchant. In that case, the merchant could probably bring a civil suit to recover their loss, and might also be able to bring criminal charges (more difficult to prove). For card-not-present purchases, if the merchant collects the CID (called various things by the different associations; the three-digit code on the signature panel of a Visa or MasterCard, for example) proving that the buyer had the card in hand, then the merchant is generally protected against a fraudulent transaction.
My point here is that I agree that your brother could have a legal exposure regardless of what your mother decides to do, if the merchants don’t get paid.
Since the Captain has chosen to involve himself he may wish to seek the services of a lawyer on behalf of his mother or brother, depending on his allegiances. This question is so specific to your situation that even the lawyers on the board will probably be reluctant to guess what’s going to happen. Personally, if I were you, I’d get out of the way.