I remember very clearly reading this fic, and it's probably in my bookmarks but I can't find it.  It's eventual wincest and the one detail I remember is that the boys are in the impala and Dean has his hand on Sam's thigh, and is gently rubbing with his thumb.  Sam realizes that Dean doesn't even know he's doing it, and realizes that it means something so after a moment or two Sam reaches over and starts running his fingers over the back of Dean's neck, kinda playing with his hair.  Dean stiffens and asks "Sam?", I think, and it all goes from there.

Any ideas?

ETA: Found!  It's the first part of the Normal!Verse by felisblanco.  Link in comments.