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*hugs* ahhhh I'm so sorry, siblings from hell... my brother used to do things like that to me all the time when I was younger, ... and a lot worse. It's hard to say what to do because different things work with different people, but I guess available options are:

1. Revenge- do something similar to an item of hers or some such
Really gets the point across but its risky, since parents can get angry and she can just... get even angrier instead of getting the message and then break something else of yours.

2. Ignore it like you don't care
Very hard to do of course, but she did it so you'd react, so if you make her think it doesn't matter all that much then she might not do it again.

3. War...
Small bits of cunningly planned revenge, but usually the other sibling does something in return, and it just keeps on going until one of you wins. Gotta keep parents out of this one.

4. Parents
Might be hard if you aren't the favourite, but if you cry lots and get the point across that it meant a lot to you, it could work. Say even though it was a small thing, you had spent ages on it and it meant a lot to you, and that her doing that was just bitchy, mean and spiteful.

5. Talking to said sibling
Don't start off angry but just really really amazingly deadly serious. You have to keep your cool with this method, and just talk to her and ask her why the hell she did it and to please not do it again. Not really guaranteed to work but its the safer approach.

6. Find a new hidey spot
Might be worth investing in some sort of locked box with a key. (Made of re-enforced steel). I really had that problem when I was younger, my brother would just come in my room and take money and torches and everything. Still does it now with blank CDs and DVDs. Basically, what they can't find, they can't destroy.

7. Nothing...
What I did most of the time, except for running to my parents... your sister will have to grow up sometime, I guess sometimes the only thing you can do is wait. It's a pain in the a** waiting, but it will happen eventually.

8. Upstage her/ make her feel stupid
Get an A in something at school, just do something awesome... does nothing to help the situation but you feel pretty damn great afterwards. Probably not recommended if you don't want anything else to be destroyed xp

Well I hope that helps, that all came from the years of experience of being the youngest and then middle child stressed worth a try at least.

OPTIONS!

1. BRING IT WITH YOU wherever you are going. put it in your locker at school, whatever.


2. GET A SAFE. with a combo/key. I'm talking a real safe here. you can put anything it it...


3. THIS IS NOT AN OPTION, IT IS A NECESSITY: TALK WITH YOUR PARENTS
both of them. at the same time.
I don't know your issues, so I can't write up a speech, but you need to show them that you are a responsible adult. don't bring your sister into it, don't let them try to reason, just talk.

set aside an hour or so...I get the impression that she does things like this all the time. you need to have a serious discussion with them.