Is it sharing to allow others to take what isn't yours without asking or is it stealing?

I am in High School, I work and buy my own clothes, I also pay a small amount of room and board to my parents. My older sister is in college, she does not work and when she did, refused to pay room and board. She often wakes up before me, sneaks into my room while I'm sleeping and takes my clothes. Sometimes i wake just in time to see my dress, jeans or sweater going past me. My parents tell me to share and they won't do anything about it and I'm furious, I say since I buy my clothes, my sister needs to ask me if she can borrow them and respect my decision. The most recent time, I'd bought a dress on payday and intended to wear it during the next out of uniform day, I earned an out of uniform day by selling things during the schools fund raising drive and wanted to wear the new dress I bought that day only to see it float past me when I woke up. I am helpless, I have no place to hide my clothes and I shouldn't have to announce what I'm intending on wearing the next day so nobody 'borrows' it, not that I think even that would help, the more I want to wear something, its always that item that disappears before I get a chance to wear it. Is it asking too much for her to ask? Of course, untill she respects a no, I doubt I'd ever want to agree to share my clothes, its just gone way too far already.