Perhaps it's this fic?

http://spnkink-meme.livejournal.com/98383.html?thread=38012239
Title: Nephilim
Author: S. Lunne
It turns out Dean and/or Sam are children of Michael. When the angels discover that the boys are actually nephilim they don't react the way Dean and Sam expect them to. Angels are extremely protective of their children and by angelic standards Sam and Dean are quite young, so the angels take to treating them like fledglings.

Despite Sam and/or Dean's protests there is lots of cuddling, and coddling, and generally treating them like children. For once the angels aren't trying to kill them, they become fiercely protective (maybe smiting anything that tries to hurt the boys.)

Thanks that is the one I was looking for

Don't suppose there's a finished version somewhere?

Just wondering if you've seen this one
Angels Among Us
http://archiveofourown.org/works/1323451/chapters/2754565

Cas and Gabriel are two angels living among the human race. Michael has decided to make use of the angels drawn to human life, ordering them to play host to the few nephilim unable to make permanent homes for themselves. These hunters are forced to live nomadic lives as they protect humanity from evil. Two of these nephilim roam into the angels' town and they're forced to provide them with a place to stay... only days away from Heaven's nesting ceremony. The Winchesters are unlike any nephilim ever created, blessed by the two most powerful angels and able to unbind the seal on their wings by sheer willpower alone. Michael fears that willpower will also unbind the seal over their angelic abilities. To keep that from happening, he tries to force them into nesting with a more powerful angel beneath his command. The only problem is... the Winchesters have found interest in their hosts, who are most definitely not under Michael's thumb.