It's not like John expects Sherlock's family to be exactly like him. Merlin knows what would happen to the world if that was the case. So, if anything, he's almost glad to hear they're not the same, and all he gives Sherlock is an amused smile.

Sherlock seems concerned about the strangest of things sometimes.

"Well, I don't think I'll run off somewhere on Christmas day," he says, even if he's not sure he would really mind at all. "But I can pop by a couple of days before it."

No problem at all. He thinks he'd be glad for it. Maybe get a break from all that pre-Christmas running about and stressing.