About half a year after Mycroft's sixth birthday, his parents informed him that he was to become an older brother. Mycroft had classmates who were older siblings and he had some idea of how the whole thing would play out, so he nodded at his mother and father and resumed reading Little Lord Fauntleroy.
As the months went on, his mother's stomach grew and grew until one evening she was rushed to hospital, and returned a few days later with a tiny pink bundle. His mother told him the new baby's name was Sherlock, and while privately Mycroft thought that was a funny name, he acknowledged the existence of his brother with a quick nod and then buried his unfortunately large nose in A Neverending Story. He was not really very interested in the baby. His classmate Billy told him that babies were loud, wet, and smelly, and even at seven Mycroft was very particular about his appearance and did not want any of the wetness or smelliness to rub off on him.
(Billy was one of Mycroft's preferred classmates; he didn't say cruel things about Mycroft's appearance.)
Sherlock cried a lot. It got to the point where their mother refused to answer his wails, instead opting for a glass (or a bottle) of wine and earplugs. One night Mycroft had had enough, and stormed into the nursery where a nine month old Sherlock lay.
"Sherlock," he said in as stern a tone as he could manage, "Do shut up." The baby didn't listen. "I've a very important test to take tomorrow, Sherlock. I need to sleep." If anything, Sherlock cried harder. Mycroft sighed heavily, and moved to sit in the rocking chair near the bassinet.
"Two by one is two, two by two is four, two by three is six, two by four is eight, two by five is ten, two by six is twelve..." Sherlock began to quiet down, snuffling and whimpering inquisitively. Mycroft continued practicing his basic sums, and by the time he got to the eights, his brother was asleep. Satisfied and exhausted, the nearly-eight year old went back to bed.
Every night Mycroft would go to the nursery after Sherlock had been put to bed and either read, conjugate French verbs, or recite sums to put the baby to sleep. Their mother cornered him one night and demanded to know how he was getting the baby to sleep, but when she tried reading from Peter Pan, Sherlock screamed bloody murder. So Mycroft resumed reciting his lessons to his brother before bed.
Coincidentally, the year Sherlock turned one, Mycroft received the highest marks in his grade.