Stepsister stealing ??

So I live with my mum stepdad and his stepdaughter. Personally I think my stepdad and stepsister aren't very normal sometimes and she especially is very sneaky! Let's call my stepsister L for short. L moved with us about 3 years ago because her relationship with her mum and stepdad wasn't amazing. My mum does a lot for L and buys/gives her lots of things. But the problem is that L steals a lot of out things. For example, my mums Tiffany bracelet went missing and we found that she took it. She kept denying it until she finally broke and admitted to it. I think it's something psychological that makes her do these things as it seems she is always taking the things she knows means the most to us. Recently I cleaned out my wardrobe and saw L spot my Michael kors watch when tidying all my things. Next day, it's gone! I don't mean to point the finger but she has a reputation of stealing and she knew where the watch was. How can I approach her/ catch her in the act to make this stop?