Muggle born magical sibs?

[Donna]

No: HPFGUIDX 130167

Turning to those in the know, is Colin Creevey muggle born?  I know in CS,
US version, page 96, chapter "Gilderoy Lockhart", Colin introduces himself
to Harry and says: "I never knew all the odd stuff I could do was magic
until I got the letter from Hogwarts.  My dad's a milkman, he couldn't
believe it either.".   Then in GoF, US version, page 174, chapter "The
Triwizard Tournament", Colin says:  "Harry, guess what?  Guess what, Harry? 
My brother's starting!  My brother Dennis!".

If Colin is muggle born so would his brother be.  Isn't it a bit unusual for
sibs from muggle families to be magical?  If sibs from muggle families can
be magical, isn't it possible for Petunia to be magical, too, but just
reject it?  We know Petunia knows more about the ww than she lets on.  

Donna

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