Note From Personal Experience:
"Itching Powder" of the sort which can be bought in "Novelty Stores" and "Magic Shoppes" around the country is actually nothing but ground fiberglass.
While, in the short term, putting said "Itching Powder" in your sister's leotard before her recital with the Memphis Ballet might sound hilarious, especially if you're 10, I can guarantee you that the itch lasts one freakin' hell of a lot longer than the joke, and your relatives will refrain from killing you only because they want to make sure you live to regret it. A good, long time.