What? You ended it there?! This is a great idea for a story!
There were some mistakes that a beta should have caught for you; most have been pointed out. Also, if Evan was born in September 1978, he would have graduated in 1997 when Harry finished his sixth year. (My parents would have graduated with Snape and Lily, and I would've gone to school with Evan, so when people flub ages and such for Harry's contemporaries, it annoys me more than it probably should. It is fanfic, after all.)
So, anyway, fix those things, make it longer, and you'll have an awesome story. I mean, did Snape accept Evan? Did he recover? Did Harry and Evan become true brothers? What about Snape and Harry? We need to know! You wouldn't actually have to write how it happened, you could just write an epilogue and give us some closure that way without writing a novel.
Author's Response: Erm... I've had everything written down (all the dates) and it seems I'm as bad at Maths as Jo is...
1978 +11 = 1989... erm. uh. Never mind. 1989+16=1995 so at least I got it right. Evan graduates in 1995 then.
An epilogue is an awesome idea, thanks.