I can't remember if it's gen or wincest. In the story, Dean has an older brother, Jake, but he's dead and both are firefighters whereas Sam is still a hunter. Dean was very devastated by his brother's death and Sam is miserable watching his brother grieving and not knowing that Dean is actually his brother. (i'm sorry, if this sounds confusing.) Sam is very protective of Dean and he saved him from a burning house when Dean was on duty. There is a scene where Dean was on drugs and he fell from a window after his current family was burned/almost burned, I can't really remember. Castiel is involved as well, teleporting Sam into the fire where Dean was trapped and here and there. 

I've never read the end of it, I would really appreciate if anyone can find me the link.