How can i keep my sister out of my room?

She is so annoying! Shes 10 and Im 14 and its hell living with her because she thinks she can just waltz into my room and take what ever she wants while being a ***** to me. I steal things out of her room but its not enough, her clothes are ugly. My dad says that we have to share clothes but this sucks! I need to know how I can keep her out of my room, but I can't get a lock because my parents won't let me.... What can I do?!?!?