What do I do?

I'm 21 and still living at home. Reason being, I'm in college and am paying for it by myself. That being said, I pay for all of my own things. The only things I don't pay for are for rent and groceries. Now to the main point.... Every single time something of mine goes missing, I can find it in my 16 year old sisters room. Shirts, jewelry, bathing suits, nail polish, literally everything.... All of which I buy myself!!! Well tonight of course I found something AGAIN and asked her why it was in her room and she went on to say "I don't know" and smiled about it. Well I can surely tell you I've never worn it because I JUST bought it. So I told her to stay out of my room and walked right back up to my room. My stepdad follows me and tells me I take stuff of hers all the time even though I have NEVER taken anything without permission and she knows that and that's exactly what I told him. Then he tells me that if I'm gonna be a bitch about it that I need to buy a new lock for my door and I said "might as well." He then proceeded to tell me that if I did that, that I would be paying rent. I am PISSED. She's not gonna sit here and take what I pay for with the money I work for, and then me get bitched at about it. Not working for me. What do y'all think would work best on keeping her out? I tell her atleast once a week to stay out of my room cause that's how often she goes in there. And when she rarely asks, I usually let her borrow stuff. But she NEVER asks. I'm about to say fuck it and but a new damn door lock. Ugggggggh. 😡 Rant is over...