My sister keeps messing up the Christmas tree!?

My lil' sis is 5 years old, and she's been doing the same thing every Christmas. When my family puts up the tree, she goes back and totally rearranges it, in a not-so-pretty way. She will move all the ornaments around, putting about 5 of them on one branch. The tree looks horrible. Whenever I fix it, she messes it up again when I don't look. I worked hard on the tree, and I don't want her to do that anymore. I've talked to her, so have my parents, and even told her that Santa won't bring her toys, but she knows I'm lying. She's usually a good kid, but for some reason, she always does this! What to do?