“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.