"But they weren’t just his mother’s parents ... they were obviously his aunt’s, too. We know Lily was a Muggleborn, from the historical records, so her parents must have been Muggles. But were they? What if they were actually Magicals, and for some reason were pretending not to be or even actively avoiding their heritage? Or what if Lily’s grandparents were Magicals, and they children born Squibs? We also assume that the Dursleys weren’t a Magical family, but Harry’s uncle knew about magic and was accepting of it, even if he despised it. Did he get all his knowledge from Petunia, and why would she tell him if that’s true?"