Family does not begin and end with blood.
A friend of mine recently confided to me that he thinks it's sometimes easier to walk away from family than friends because you had no hand in choosing your family and oftentimes find yourself with nothing in common with said family. The sense of obligation towards them comes from tradition, from rote, and also often from guilt.
But he was using family in the most traditional sense. Families, to me, do not merely include those to whom you are bound by blood and genetics, but rather they are the groups of people with whom you have a deep bond and spiritual connection. A family is who you turn to in times of crisis-- who you know you can always turn to when things get rough. Therefore, I think it is easy to argue that the strongest family bond on television right now is the family of misfits on Supernatural.
Of course, Sam and Dean are actual, blood-related brothers, and we have seen them fight and die for each other quite a few times before. But they have an extended clan who-- although they don't get to see perhaps as often as they'd like-- never hesitate to pick up a gun or a flask of Holy Water and get in the good fight, too. The family was larger once, but though it dwindles in numbers with every passing season of the show, it never seems to come up lacking: Bobby, Ellen, Jo, Castiel, and some could even argue the Ghostfacers. These are the kinds of people I'd want in my corner!
Though for those same reasons, I admit Dean kissing Jo (which for some fans was, like, "Finally!") was a bit like watching a brother kiss his sister and therefore left me with a touch of the "ickies" instead of the "awwww's!"