I'd demand my right to privacy, even though I personally did not pay for the house... My humanity is enough for them, isn't it?
I've actually had my little brothers snooping around in my room several times, though nothing bad happened. I didn't let them know I knew, shrugged it off, and made sure everything was safe... Not that I... Okay, I did have something to hide all those times. Actually, several somethings. Be assured, they weren't dangerous and/or illegal somethings, but to have them know, what a scandal, even if it were only between them and I... I'm very sensitive about my pride.
If what's (if anything) happened has greatly upset you, then i say maybe confront your parents about it. Or raise money for a lock, or maybe a doorknob that locks.