When Sam gets hit by the curse Dean dies in the melee and haunts the motel room where Sammy lives in the walls.
Ooof, what a painful story idea!
Sam would be torn if he knew Dean’s spirit was trapped in the motel. He’d want his brother to be free, but also Sam doesn’t want to lose his only connection to his family. It would rip him up inside no matter what decision he made in the end.
I don’t have any stories like this for BA, but I did do a short story with the Impala and Dean connected, and there’s also a fanfiction for BA where Sam died when he was cursed as well, so check them out!