Oh, nice one. I like how all the characters have their prejudices, but they're not monsters -- just flawed, complicated people. I think you captured Petunia really well in this line: She'd have told Albus Dumbledore to leave her family alone. This is just right, because although Marge isn't someone most of us would admire, Petunia would, and this is exactly why. Petunia sees her as strong, and she feels that she wasn't strong enough to keep her family safe from dangerous influences.
Dudley snorts and heaves himself off the bed. "Good luck. I've been trying to sort through it since we were eleven."