We Three Kings (http://cosmic-medusa.livejournal.com/10326.html)

Cas, a doctor, falls in love with the mechanic, Dean Winchester. He understood, early on, that this meant adopting his younger brother, Sam. He didn't, however, count on all the loss that would follow the Winchesters, the addiction that would trouble Sam, or the problems his own family would cause following his commitment to the boys.