I am going to visit in the mountains but afraid what to pack because my cousin steals?

I am going to visit my family up in the mountains but my mom says that I have to pack light because my cousin (who is close in age to me) likes to steal things. She has stolen makeup, clothes, and other things from me in the past. I an usually not a light packer so it's going to be really hard to pack light. But I don't want her taking any of my things. What should I do?