jk at book festival answered this question
audiance member- Is aunt Petunia a squib?
Rowling- Good question. No, she is not, but—[Laughter]. No, she is not a Squib. She is a Muggle, but—[Laughter]. You will have to read the other books. 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.