OOP: Petunia

[Andrea]

No: HPFGUIDX 61903

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Having tried to search the archives for a similar idea and coming up empty, I would like to suggest:

Aunt Petunia is not a muggle, not a squib, but a full-fledged witch.
Canon?
Here we go:
Remembering something that your "estranged" sister said to you 14 years ago in a blink of an eye [Dementors](page 31 US)? Sounds fishy.
Understanding the full terror of LV (page 38 US)? Not very muggle-like.
The kicker:
"The furious pretense that Aunt Petunia had maintained all Harry's life -- that there was no magic and no world other than the world she inhabited with Uncle Vernon -- seemed to have fallen away."

Yes my friends, a witch. Did she go to Hogwarts? did she stay through OWL's or NEWT? Dunno.
Did she _choose_ later (or early) in life (before meeting Vernon) to leave the WW, break her wand, and never speak of it again?  Perhaps.
OR
I tried looking up to see if we know Lily was an older or younger sister. If Lily was older, their parents might have forbid Petunia to go, even if she received a letter, if they thought Lily was a freak?
Could be this also.

JKR said someone will _develop_ magic later in life (hope I've got the quote right). 
Develop: [1]To bring from latency to or toward fulfillment (not like Develop: [2a] To come to have gradually; acquire: [2b] To become affected with; contract; developed a rash)
Although 2B might be what Petunia thinks of it (rash indeed). 

So although from our good friend Dudders to everyone else in HP world has been theorized to be the one with magic to develop, I go with Petunia and our newfound canons.

-Andrea/Kattrap