Been going through AO3 tags and going no where ^^
Not sure if it was a Sam/Dean or CastielDean or just Gen.

Sam finds out from John that he has a brother and has been institutionalized or so. Sam get mad and takes Dean out. Mainly remembers that Dean can play the piano even though he never did before. Sam gets a professor from his school to come and see. He talks to Dean about why the music sounds wrong or so and that Dean listens closely to the sounds to make them sound better.

I believe it is a WIP or unfinished fic.

Thank you for your time and help.