Love Sherlock’s reactions here. His whole life, his brother has been making him feel like he’s less than, stupid, unworthy, weak…typical elder brother stuff but with the added problem of Mycroft being a genius who can assemble the perfect put-down at a given moment, to maximize Sherlock’s agony. Sherlock probably gave up long ago on trying to fight back.
Mycroft delivers his zinger, Sherlock is looking at the floor, round-shouldered, bracing for impact. John bats the threat out of the air without a second thought; of course he’s completely unafraid of skinny, brainy Mycroft because Mycroft poses ZERO threat to him, like come on, let’s not kid ourselves. John vollies back, Sherlock raises his head, looks at John…Oh, right. You’re not afraid of him. He can’t hurt you like he can me.
Then Sherlock laughs, partly because John has bested the brother who has always made him feel small—in a war of words, no less!—and partly because he’s finally, finally got someone to back him up. Sherlock spent his life apart, alone, without a “gang” (only grudgingly, he and Mycroft were their own gang by default, but neither tolerated the other unless absolutely necessary to join forces to ensure survival).
John has just reminded Sherlock: I’m on your side. We’re a team.
even though I think Sherlock can give as many insults as he receives from Mycroft, I wholeheartedly agree, I love it when John so clearly takes a stand for John - especially for Sherlock as the younger brother :]