My advice, when your able to forge your own path, leave them to rot. They clearly don't respect you if their response to stealing money is finder's keepers. Doesn't matter where it was at the time. Your brother is in for sharp shock I imagine when his mother can't protect. That kind of carefree attitude always bites back in the end.

Also petty revenge isn't the solution, as some have stated. Focus on yourself, and being self sufficient. That way when the time comes and they beg you for money. Just give them a middle finger.