Mycroft was born a miniature adult, mature and well-behaved and dreadfully, dreadfully polite.
Sherlock was what we may kindly term a difficult child, every day a temper tantrum or an upset neighbour or glowering black silence.
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Mycroft never received anything less than a perfect grade, naturally.
Sherlock achieved a rebellious mixture of As and Es, depending on whether he thought the subject matter worth remembering and on how many experiments he had going on the side.
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Mycroft had a group of friends all very similar in nature – rich, double-barrelled and well connected.
Sherlock wasn't bullied – his brother saw to that – but there was never really anyone that he would have considered as a friend.
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Mycroft graduated with a First from Oxford and was immediately head-hunted by several important government officials.
Sherlock was kicked out of Cambridge halfway through his second year for exposing one too many extra-marital affairs amongst the Fellows of his college.
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Sherlock lives in a cluttered but cosy flat with his closest friend and spends most of his days running around the city and doing what he loves best.
Mycroft eats a solitary sandwich at his desk, and lives alone.
Apologies for the random full stops. I wanted to separate each little couplet, but the formatting is being weird and it was the only way I could do it.