The things with some elders is that they never grow the eff up. They lay an egg and will always see you as that egg but reality has it that you already bloomed into a chicken that showers your private parts daily.
It is this entitlement that is bothering you. There is no ground of your property because what is yours is theirs first. They failed to see that you are not their little princess anymore and thus controls you at all cost.
You should consider moving out and let your other siblings to tell your own folks to why you wanted to move out on grounds that they do not respect you as a grown individual. I'm not saying that you doing this will teach them a lesson but if it must then do it cuz this is the western world where we have unspoken rules and culture shock for all to learn.
I feel you on this. My dad opens up all my mails. It bugs the chit out of me. Opening someone's mail is a violation of my privacy act. However, I gotten used to it cuz well, it may not be his choice to do thus cuz the guy has alzheimers. Even if he reads my letters, I wonder if he even knows what is going on. What am I suppose to do? Scold the poor guy and then what? He is a ticking time bomb and waiting to die. You don't want to strike a person and the next day they might die.
My situation is different than yours but the underlying statement here is for us to understand the nature of the situation. What is it that we can do without? While some are offended, others do wishes that they even have a set of parents who are that nosy. You are lucky to have parents that are still caring. Besides, what you have of that importance that you must hide? Are you famous? You a nasty freak? The fact that you told them your "stash" is your mistake. A secret, no one should know. If you tell, they look at it, I'm favoring your parents here.