Is Petunia magical?

[Bruce Alan Wilson]

No: HPFGUIDX 160435

JKR has said that as far as Dudley goes "what you see is what you get" and that
Petunia  (a) is not a Squib and (b) is not the 'person who will use magic late
in life.'  Personally--and I know I've said it before--I think it will be Uncle
Vernon; it would be such a kick in the pants for him to find out that HE has
wizardling blood, too.  I can imagine him carefully examining the genealogies of
Dudley's girlfriends. (Yes, I'm sure Dudley will have a few.  As my
great-grandmother used to say, "No pot's so crooked that no lid will fit it.")
 

BAW