By not presenting solutions to get your money back and not telling your parents it's time to compensate and not filing a police report, you and your sister are enabling his theft and letting him know that he can get away with it in the future. If you're willing to be sneaky, your next best revenge is to delete his Fortnite account. You may need a security code for this. That would hit him HARD if he's like any of the Fortnite players I know. Your mother is actively causing trauma to you and your sister. Stand up for yourself and your sister and do something about it.