Sister keeps taking my stuff?

I'm 15 and my sister is 13 and whenever I leave my room for, like, a second she goes in and takes stuff and my parents don't do anything about it. I've tried hitting her, pretending I don't care, hiding my stuff and all that! It's not like she idolises me or anything because she hates me and calls me names and hits me, and my parents find it funny but when I hit her, they go mad because 'I can hit harder than her' and it's not fair, they assume because she's younger she's always the victim in our fights and I can't stand it anymore it's ruining my social life I can't go out with friends because I have to stay in my room 24/7 to guard my stuff!!! Any advice?