Financial talk

I wasn't sure if I'm in the proper area for this, but here goes. My SIL's boyfriend has been stealing gas from a local gas station down the street. So far, he has admitted to doing this twice in the past week because he doesn't have the money to pay for it. My SIL is very distraught about it because she is afraid he is going to get caught while they are drunk. (They are both underage!) I care a lot for my SIL and I don't want to see her get in trouble. Should I call the police and report her boyfriend or should I just let it go? This isn't very fair to anyone else who has to pay for gas.