The fic I'm looking for has Dean as omega, I think, and he's basically stayed at home his whole life to look after his family (in this case, Sam and his dad, both, I think, who are alphas). After Sam goes to college, Dean decides to move out to try living on his own. Partly, I think, because he's looking for a mate but also because he wants to spread his wings now that Sam is no longer living at home. He ends up rooming with Castiel, who ends up triggering Dean's first heat, I think.
The fic was very low-key (not a lot of sex, if I remember correct), and it wasn't very long, either. It's obviously AU, and the brothers aren't hunters. I haven't been able to find it here using the appropriate tags, but that's not to say I didn't just miss it, of course.
Ring any bells?