I read this tale some time ago.  It takes place mostly during Devil's Trap and possibly Salvation in the canon timeline.  John is haunted (possibly already possessed) by the YED, and he takes John, following Sam and Dean and reveals their Wincestuous relationship to John by having him watch them outside motel room windows and such.

The final scene (in the cabin), has John/YED confronting the boys in a not-so-paternal sexual manner.  Hotness ensues.

I sure hope someone can help me find this fic.  It's about 2 years old.

Thanks in advance,

Glim