sardonicsmiley's Five Ways Sam Winchester Wasn't One of the Demon's Children (http://sardonicsmiley.livejournal.com/8813.html#cutid22), and sequel Three Ways Dean Winchester Was Saved by Someone Who Loved Him (http://sardonicsmiley.livejournal.com/8813.html#cutid23). You might also enjoy the author's Hell 'verse - both have powers, though not since birth.

Phantisma has an AU that deals with both having powers-

Ages- summary- This is an AU story that begins when Sam is 8 and Dean is 12. It is primarily told from Dean's point of view, though as it progresses we get to see parts of it from Sam's POV and even John's.

Overall summary: John is away on a hunt and Sam is sick. Really sick. Dean ends up taking him to the emergency room, and in the aftermath of that fateful decision, both boys end up in foster care, where they try to make a life for themselves. But, they're still Winchesters and find themselves caught between two demons fighting a war that will only end in massive destruction.
http://phantisma.livejournal.com/92878.html