How can I protect private folders on my Mac?

Seems to me that the very best possible solution is for you and your sister to set up different accounts on your Mac, actually. That's one of the real benefits of Mac OS X (and, for that matter, Windows too).

You can do that by simply going to Apple --> System Preferences... --> Accounts and setting up an account for her. Then simply ensure that you log out when you're done with the computer each time and she won't be able to get to any of your files and you'll be safe from having her mess with your preferences, settings, etc.

No? That's not what you want to do?

Okay, I'll give you a couple of neat Terminal tricks that can help you out, one that lets you make a folder closed to everyone and another that makes it disappear completely until you make it reappear.

In both cases, you'll need to launch Applications --> Utilities --> Terminal.app. It'll start out in your account "home" directory, which you can check by typing pwd. Move to the directory where your folder or files lives by typing in cd followed by the name of the directory.

For example, if I wanted to protect a folder on the Desktop, then I'd type in cd Desktop and move there.

Now, to make a folder completely locked and unreadable, simply type in:

You won't get any feedback but you'll end up with something like this (pay particular attention to the "testing123" folder and how it's portrayed):

To get back in when you want to, simply go back to the same place with the "cd" command and type in:

To hide a folder or file completely, simply preface its name with a ".". For example, to hide "testing123" I'd rename it ".testing123":

Wanna get that one back to normal? Go back to the same directory and type in the reverse: mv .testing123 testing123

So there you go, three possible solutions to your problem. Hope one of them works out for you!