To anybody who happened, if by chance or twist of fate, to meet Sherlock and Mycroft Holmes, the complete lack of affection between the two brothers wouldn't be difficult to perceive. To the eyes of a common person, it wouldn't look so dissimilar by a quite common brotherly rivalry; to a more perceptive man, though, the contempt and disdain of the older brother would be obvious.
There was a time, in the years of Sherlock's childhood, where Mycroft would have admitted to care deeply about his little brother: yes, he claimed to be the clever one and more than often would be rude towards the kid, but that was just his temperament.
Yet, Sherlock cannot recall a single moment in his life, after the accident, when Mycroft had shown signs of brotherly affection towards him. In the accident he did not only lose his beloved dog, Redbeard, put down to avoid him suffering, but apparently any bond with his big brother.
If you asked Sherlock Holmes, He would say he couldn't care less. Yet, he's a good liar.
"There will always come a time when we need Sherlock Holmes".
Mycroft Holmes controls the MI6, U.N.I.T. and basically the whole British Government. If any problem big enough to potentially harm the United Kingdom arose, he would be the first person to know and accordingly to act in order to prevent any disorder.
Which is why, when U.N.I.T. needed to give a new identity to a powerful enemy of not just England but the whole human race, Kate Stewart asked Mycroft to control him personally. He would be given a new identity, she said, a camouflage so perfect he would even forget his real identity, provided he never opened a fob watch, secured and hidden in the Tower of London.
"We need a background story. Last time it didn't work as well as we thought: he still knew he didn't belong to that place, or at least to that time…"
Changing memories is an easy thing, especially for the man in the Blue Box: changing the memories of Mycroft's parents in order to make them forget about the accident was so easy: almost as if they wanted to forget it, not that surprising considering what happened.
But if the memories can change, the mind cannot: so Sherlock Holmes, who was a very simple and ordinary child, became the genius and the clever man, fascinated with mystery and murder, drawn to the challenge of the mind and the thrill of daring.
And Mycroft Holmes kept watching the man who pretended to be his brother through cameras and spies: alert to any change, to any symptom of danger.
"One day we will standing around a body and Sherlock Holmes will be the one who put it there".
If any act of violence or sign of psychopathy would emerge from Sherlock Holmes, Mycroft knew what should have been done, and it was to reset the procedure completely: open the fob watch, make him regenerate, transfer him to another place and time and give him a brand new identity.
Mycroft actually initially waited with eagerness for that day to come: to get rid of the burden, of the pain of acting, to get rid of that heinous being who dared to pretend to be Sherlock, and to stop having to watch his parents to care about a man who didn't deserve it at all.
But then, Sherlock met John Watson, who somehow was able to take the best out of him: to make him a good man, not just a great one. And Sherlock himself started caring for people.
"Caring is not an advantage".
And while saying so, Mycroft would reprehend himself: do not care for him, do not start caring. Time would pass and Sherlock would fake his death to stop the consulting criminal James Moriarty: everybody believed him dead, apart from those who controlled him.
Mycroft could have let him die in Serbia, but instead he climbed the military rank just to take him away from that hell and in London again: to foil a terrorist attack, he claimed, but Mycroft Holmes was too a good liar. Because seeing that man care about John, about his landlady, about an inept Police Officer, made him seriously wonder if caring was so much a disadvantage.
"Your loss would break my heart".
He started seeing Sherlock in that man.
And when the final act was made, when that man willingly shot Magnussen in the head on that awful Christmas Day and kneeled in front of the pointed guns and the headlights, Mycroft didn't see a killer: he didn't see the Master in a prison of humanity and fake memories, he saw a little kid.
He saw his brother on the day of the accident, when he lost his life trying to protect his dog from an incoming hit and run. Crying.
"Oh, Sherlock. What have you done".