At a glance, it would seem they are...until you factor in their background.

Dean is a parentified child. He was Sam's primary caregiver under difficult and dangerous circumstances. He learned early and hard not to take his eye off the ball(Something Wicked). We see the line between parent and brother blur with both action and word - mostly in earlier seasons, but even as late as Dark Dynasty. And parents don't outlive their kids.

Sam will always be...well, the Child. Constantly running either to or from Dean. Clandestinely scheming and secretive. He wants so badly to 'save' his brother, to make Dean proud, he misses the forest for the trees. But he never stops trying, that validation means so much.

So, I don't think codependent. I think they are each chasing their own tails while orbiting the black hole that is John.