Racheling
If your siblings are really stealing your stories to turn in as their own homework, you should tell your parents and/or your teachers. That's 1) not fair to you, and 2) academic dishonesty and plagiarism, and they could fail their assignments if they get caught. At the very least, maybe find a way to hide your stories, even if you share a computer with them.
Flash Drives/Memory Sticks are nifty little things. Save your documents on it instead of the computer, and then put the drive/stick somewhere only you can get to it. Like your front pocket. Or a lockbox in your room. Or inside your pillowcase. Somewhere they wouldn't really think to look. The same can be done with a rewritable disc.
Or even save it to the computer, email it to yourself, and then delete it from the computer so only you have it.
Phade is paranoid. She does these things often. Not so much to prevent theft as it is to prevent accidental loss should her computer crash, die, or need to be wiped.