Lily's Parents

[karen]

No: HPFGUIDX 170072

I have been interested quite some time with the case to be made for 
Lily and Petunia not being sisters, the main support for this being 
that in book 5 JKR draws attention to their eyes being very 
different, but now I have remembered something else that might 
support my prediction.

I'm re-reading book 2 right now and in chapter 9 the students are in 
history of magic and Professor Binns is telling them about the 
founding of Hogwarts.  "A rift began to grow between Slytherin and 
the others.  Slytherin wished to be more selective (in itallics) 
about the students admitted to Hogwarts.  He believed that magical 
learning should be kept within all-magic families."

Now the significance of this statement is that Harry was considered 
for Slytherin in book one while wearing the sorting hat.  If he was 
considered then he must be full blooded.  We already knew he was 
considered at school to be from a all-magic family but what is an all-
magic family?  If Harry's grandparents were muggles, as we suspect 
they are, then wouldn't he not be from an all-magic family?  To be 
considered for Slytherin he must have been full blooded which means 
that either Petunia's parents are not Lily's parents (Lily parents 
would be a witch and a wizard) or Petunia and Lily's parents are not 
muggles.  What do you think about those possibilities?