Here's the thing....
If you don't press charges, then you just reinforce the fact that he can get away with anything and that there are no consequences to his actions. Stealing $40,000 from anyone is a major fucking deal. But stealing that kinda money from your elderly mother.....that's lower than the lowest. You aren't doing him or anyone else any favors by letting him get away with it. I realize it's your brother, but I too have a brother that's good for nothing. My Mom enabled him until he was 30 years old. Once she finally said "enough is enough," he was faced with being homeless or joining the military. He had never even had a bank account until he joined the Army a few months ago.
Bottom line, if there aren't some major consequences for his actions, then he'll never learn. He's 53 for Christ's sake. It's time he learned something