I read this Fanfic sometime last year and have been searching for it for about 3 months now!

Dean and Sam return back to a cabin (in a swamp possibly) where they stayed a summer as children. During the duration of that summer Dean is molested. In one of the chapters (a telling of he events) it is described as being hot and sticky as Dean is wrapped in a blanket sleeping on the floor while Sam sleeps on the bed. Dean is only abused when Sam is sleep. Dean begins acting peculiar and distraught.
Sam does not know about any of this until he peers through the Cabin door and sees that the man is going to rape Dean again. Sam is enraged and shocked. He then knocks the door down and attacks the man. Dean is now catatonic or acting like nothing happened. Sam tells Dean that they need to talk about (I believe they are in a kayak-boat now). Dean dismisses/objects/declines Sam at first but then tells him what happened. The narration alternates between NOW (current events) to THEN (Sometime in the 80's/90's.)
John is not involved as he left them there while he was on a hunt. John is friends with the man who is possibly a hunter.