The first time that Sam and Dean get caught by someone who knows them, they’re faced with the horrified, “But you’re brothers!”
Sam swallows, trips over his words. He looks to Dean, needs a cue. Dean clears his throat. Sits up on the bed. With the blankets around to cover them, he elbows Sam. Sam tenses. Readies to bolt.
Then Dean grins.
"Hey, Sammy. Butt-brothers."