It happened so gradually, that even Mycroft didn't catch on at first.
At first, there was Lestrade. He seemed to take some interest in Sherlock's life beyond the cases they worked on. There were the drugs, which the DI was relentless in pursuing and eradicating from Sherlock's posession. Over time, he formed a a somewhat familial connection with Sherlock, an uncle of sorts.
Then came Mrs. Hudson. Her style was to mother the stuffing out of Sherlock, and curiously enough, he seemed to love it. He even returned her affections, effusively. High-functioning sociopath, indeed.
Molly was harder to define. The consulting detective was comfortable with her, and acted in a familiar manner. Not quite a sister, but perhaps a close cousin.
Then came John. There was no doubt about it. Sherlock and John were brothers. Very close, very devoted brothers. Astoundingly, the social misfit had managed to build himself a new family.
The consulting detective's blood relatives weren't completely out of the picture, but they were more distant and estranged than his chosen family. The elder Holmes's had always tried, but in the end, they were far from understanding their younger son. Contact between them remained warm, but very limited.
Uncle Rudy was never much of an uncle, even when he wasn't suffering from dementia, as he currently was. Sherlock wasn't even aware of the existence of his sister, which naturally precluded any contact at all.
Then there was Mycroft. The brother who tried. He never gave up trying, in fact. They had a difficult relationship, fraught with hostility, mistrust, and misunderstandings. And yet, Mycroft kept trying, hoping to one day be the brother for Sherlock that he once was.
Dr. Watson was both a help and a hindrance. He encouraged Sherlock in his path towards finding his humanity. He also mostly viewed Mycroft with suspicion at best. Sometimes, he would chide Sherlock for his childish feud with his brother. Other times, he would join in, taking Sherlock's side, naturally.
Sherlock jumped from St. Bart's for his family. Lestrade, Mrs. Hudson, John. He would have done it for Molly too, had Moriarty known she counted. He didn't do it for Mycroft.
Sherlock killed a man and risked imprisonment, even death, for John, Mary, and their unborn child. His family. Mycroft was left to pick up the pieces, and then summarily pushed out of his life.
Sherlock forced his brother to sit in the client seat. It was childish and trivial, payback for his lies. To Mycroft, it was also significant. When he said, "John stays," It was significant to John. To Mycroft, it was merely a verbal declaration of what he had known all along.
If Sherlock had replaced him, it wouldn't be too difficult to get him to shoot someone who was practically a stranher, and a strongly disliked one, would it? He just had to help him along a bit, as it appeared that Sherlock did have some sort of conscience under his assumed sociopathic persona.
Mycroft tried. It didn't work. He was left to wonder, in the damp, empty cell; was Sherlock too possessed of morals to shoot an innocent man? Or was there something left of their fraternal relationship, after all?
