Croatoan works for me! It's still canon, but keeps it simple. OOOOOH, plus they'd all be at risk, and they wouldn't know Sam was immune. Until someone tries to infect him and it fails.

Oh yes the annnggggggggggssst on missing everything. Piled on top of normal sibling rivalry, haven't seen each other in years, history of Dean flirting with Sam's high school girlfriends, fucked up sexual tension with Dean, simultaneously terrified of the baby and heartbroken that he missed milestones ... all while Sam attempts not to act like a jealous/protective caveman.
Plus zombies keep trying to eat them. Y'know.

Where do you think John is in all this? Missing? Dead?