Just an update: I found my eyeshadow palette. My little little sister hijacked it and I found it when I knocked a purse off of our purse rack in our room.
Now, the question is, do I approach her directly about it or do I just "miraculously" find it somewhere obvious, like under the bed or in a bathroom drawer? I think we'll just wait it out and see how she reacts. What was she going to do with it, anyway?