Phantasima has some great long limp Sam wincest stories!
The Very Dark Wincest Verse has the first part which is-
The Good Son: A friend calls Sam back to Stanford to investigate a series of disappearances, but before Sam can start to investigate, he is kidnapped by the culprit, a man with a connection to Sam's past.

And a second part-

Where It Hurts: Kendall Garrett isn't done with Sam yet.

Both parts can be found here-
http://phantisma.livejournal.com/91891.html

She also has There Should Be Light- summary- When Sam is stolen by demons and taken from this earth, Dean finds a way to win him back...but when they return, the world is a changed place, and Sam is a changed man...and every day brings them closer to the end...unless they can find a way to undo whatever it is they did to bring hell to earth.
http://phantisma.livejournal.com/185446.html

And The Keeper Verse features a Evil Sam who is redeemed over time and we get to see lots of limp Sam even though at times he is a dominate Sam in this story- it is very well done.
http://phantisma.livejournal.com/91137.html

And her Aztec Porn story has limp Sam- summary-
Back Story: Sam went to Stanford and John disappeared, leaving Dean alone. Then Dean disappeared too and Sam is left only with a strange voice mail from his father about Dean being missing, an ancient Mexican artifact and a town in New Mexico where Dean had last been seen.
http://phantisma.livejournal.com/154964.html

Should keep you busy!