Sister needs to respect my privacy?

Ok so here's the deal I'm an 16 year old guy and my sister is 14. Well of course you know that it is perfectly normal for me to well..masturbate at that age. Anyway I was in my room just going at it and I had my door locked because my sister always tries to come in. Well she quietly uses a screw driver to open the door and I didn't hear or notice a thing until she was right behind me and screamed ewww!! which freaked me out sh!tless. I yelled at her to get out of my room and she ran out the door. My face was so red not so much because I was mad but because I was embarred. I mean she came into my room and saw my "stuff" and there was a lot there so of course it was hard to hide if you know what I mean. That was the last straw, how can I get her to respect my privacy?