The "why" questions seem to be tapering off, and she is now developing the truly endearing habit of explaining things to her brother. She can be as creative as she wants; he's not going to argue with her logic.
Yesterday I heard her patiently explaining, "I can't give you mommy milk, Zoodle. I can't give you mommy milk because I'm a racecar."
Well, as long as she's not trying to nurse him or give him gasoline, I'll be happy.