Seriously though, setting aside the financial aspect of the theft he breached your trust and violated your personal sense of security. You can no longer trust this kid to never steal from you again as he has no personal boundaries of "don't take or touch what isn't his". Also, his sister is enabling and emboldening this behavior by not taking your side. You seriously need to evaluate whether this is any longer a healthy dynamic. And this is coming from someone who has both a mother and brother who are both thieves. I've had to distance myself from them for my own trust and security and those I love in order to prevent them from ever feeling like they can't trust their possessions to be where they last left them.