It strikes me that the boss is making up the theft allegation in an attempt to have you dissmiss your complaint with the labor board for your unpaid wages. I seriously doubt you have anything to worry about.
but because his sister is a lawyer and she will probably do everything she can to have a case. The theft allegation has absolutely nothing to do with any lawyers. All the boss can do is file a report with the police, in which case the boss is going to have to give them all the specifics including the customer's name who claimed you pocketed the money. That's just not going to happen. The boss is just blowing hot air.
BTW, most lawyers are highly reputable people. Brother or not, it's incredibly unlikely she's going to waste any time on this nonsense, not in the least because there's absolutely nothing a lawyer would or could do in a situation like this.

