Because all brothers, upon reaching the age of 5, sign a pact to always annoy their sisters at every opportunity. Personally, I was an annoyance prodigy and got to sign the pact at the age of 3. Of course, no brother is allowed to tell his own sister, and will deny the existence of any such pact if pressed on the matter.
Edit: the group who created the pact is called the Brothers Of Sisters Society, or BOSS. We wouldn't use the "of" in the acronym, but it makes it much cooler.