Is Petunia Dursley the secret-keeper to the whereabouts of 4 Privet Drive?

Clues:

• JKR says that there is more to her than meets the eye.
• Although JKR says

  "Aunt Petunia has never performed magic, nor will she ever be able to do so."

  Dumbledore could have performed the Fidelius Charm and concealed this secret within her. Do you have to be a witch or wizard to be a secret-keeper?
• She obviously knows far more about the wizarding world than she pretends. (bk 5, pg 33)
• In bk 5, Arthur Weasley and Sirius attach great importance to the fact that Harry must not leave his Aunt and Uncle's house. (bk 5, pg 32,40)
• Dumbledore insists that Harry returns there each summer.
• In bk 5, just after both Harry and Dudley have been attacked by dementors, Petunia insists that Harry stays in the house. She does this although she knows that it is because of him that her beloved son Dudley was earlier in such danger. It seems that she knows no one will harm Harry while he remains in her house. Does she understand this so well because she is the secret-keeper?

Petunia Dursley and Sirius Black

We know that Sirius Black left 12 Grimmauld Place at the age of sixteen and moved in with his best friend James Potter. If James often met Lily at her house, it is probable that Sirius often visited also. If this was the case then Petunia could not have avoided either of them if she had so desired.

It is more likely, however, that she wished very much to belong in their company and tried her best to comprehend their conversations about the magical world. We are told that James and Sirius were both popular and good-looking. It is likely that they would soon have procured the affection of Petunia and caused her to wish that this affection was mutual.

It is not unlikely that Petunia and Sirius had some kind of affair which she viewed as serious but Sirius did not. If Sirius had left her when she was expecting some kind of marriage proposal, she would have understandably been upset and hurt. This may be a deeper reason for her apparent hatred of the magical world.

When it was known by everyone that Sirius Black was on the loose at the beginning of “Prisoner of Azkaban,” Petunia frequently looked out of the window, as if expecting to see him. There are a few different possible reasons for this: she was genuinely extremely frightened of him; she believed he had broken out of Azkaban for her; she had had an affair with him in the past and was worried he would turn up at Privet Drive and reveal all to Vernon.

A Banshee?

In an interview, after somebody suggested that Petunia is a squib, JK Rowling denies it but says that this is a close guess. Does this mean that Petunia is not truly a muggle but actually some kind of magical being?

A banshee is descibed in "Prisoner of Azkaban":

"...a woman with floor-lenght black hair and a skeletal, green-tinged face...she opened her mouth wide, and an unearthly sound filled the room,a long, wailing shriek..."
(Bk 3, pg 104, UK paperback edition)

We are told that other creatures such as hags exist so why not banshees?

If she is a banshee and banshees, like werewolves, are not entitled to all the rights of witches and wizards, it would explain why she has so deep a hatred of them.

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