Your parents are divorced, it was his card, it is his option to call the police and press charges. If that is what he wants to do, he should do it. He should have already notified the credit card company about the fraud.
Your mother owes him nothing, this is his problem to deal with.
Me? I'd stay out of the entire mess. Your sister is an adult, she will seek recovery when and if she is ready. IMO recovery cannot be forced.