"I’m telling you," Isaac continued, “Augustus Waters talked so much that he’d interrupt you at his own funeral. And he was pretentious: Sweet Jesus Christ, that kid never took a piss without pondering the abundant metaphorical resonances of human waste production. And he was vain: I do not believe I have ever met a more physically attractive person who was more acutely aware of his own physical attractiveness.”
“You might have got the impression that there is a little bit more to Aunt Petunia than meets the eye, and you will find out what it is. She is not a squib, although that is a very good guess. Oh, I am giving a lot away here. I am being shockingly indiscreet." - J.K. Rowling.
If the reveal had been that Petunia was a witch ~ Several years after the war, Harry tracks his aunt down to confront her with what he witnessed in the pensieve.