How about this one?

[bkb042]

No: HPFGUIDX 40552

Snape is Lily's brother.  It has been noticed that "Severus Snape" 
is an anagram of "Perseus Evans". We also have precedent for 
siblings to be split apart during Sorting (Padma & Parvati Patil). 
We have evidence that Petunia is a liar with regard to how the 
Potters died. She is also bitter and spiteful towards Harry in 
particular in much the same way that Filch is towards the student 
body of Hogwarts in general.

That said, how about this for a backstoryline: Snape intercedes for 
Lily with Voldemort. He could care less about James (not many 
brothers-in-law like the spouses their sisters choose), and Harry 
threatens the existance (albeit in the future) of Voldemort since 
Harry would be honor bound to avenge the death of his father. 
Voldemort basically tells Snape "O.K. As long as you keep Sis off my 
back.". Pettigrew and Voldemort arrive in Godric's Hollow to off 
James and Harry, but Lily is being too maternally obstructionistic 
with phase II. Voldemort has no recourse but to AK her in order to 
get to the troublesome diaper-jockey. It is at this point that his 
metaphorical gun metephorically blows up in his face. Upon hearing 
the news of his sister's demise, Snape changes sides. Harry is all 
he has left of his wizarding family (since he changed his name and 
cut himself off from his muggle parents and sister), and feels bound 
to protect him (the quidditch match), but can't help hating him 
because of James.

BTW, I know why Fluffy has an attitude: He's got six ears an can 
only scratch behind two of them! That'd be enough to drive anyone 
nuts!

bkb042