He's three when he has his first encounter with water, and it's one that almost ends in tragedy.
"Come look at the swans, little brother," that's what Sherrinford says, and he doesn't suspect; he's too young to see through his sibling's malicious intent, and he likes swans with their long necks and white feathers.
Next thing he knows he's being pushed into the river, tries to scream in panic but only ends up filling his lungs with liquid death. He wakes up in Mummy's arms, drenched and shivering with fear; it's the one time he sees Father get truly angry, and Sherrinford is confined to his room for the rest of the summer.
None of them ever mention the incident again, but he for one doesn't forget it.
xxx
He's five now, and has learnt his lesson – his brother is not to be trusted, ever.
That's why he ignores Sherrinford's cries for help as they come from the river; surely it's just another of his tricks, and he won't fall for it. He watches his brother as he struggles against the current, until he can't see him anymore.
Only then he runs home to call for help, even though he knows it's too late.
Mummy and Father don't cry, they just hold each other and struggle through the funeral. Mycroft hides under the bed and cries until his eyes are swollen and his chest aches with pain.
His parents keep telling him it's not his fault; he doesn't believe them, not even for a second.
xxx
He's seven when Sherlock is born, and he point-blank refuses to go anywhere near his baby brother.
"You don't have to be jealous, Mike," his parents chide him gently. "We still love you."
How can they not see he's trying to protect the little one? If he doesn't have anything to do with his new-born brother, then he can't possibly cause his death.
It's late one night when he tiptoes to Sherlock's crib, takes one good look at his little brother and tells him what happened to their eldest brother.
"That's why you're better off without me," he adds in a forlorn tone, realizing a moment too late that Mummy is standing behind him on the threshold.
"Oh, you ridiculous child," she murmurs soothingly as she scoops him up and clutches him to her chest. "Sherlock is lucky to have you."
And she's right, in a way. He'll always be there for his brother, will always make sure to pull him out of whatever river he's trying to drown himself into.
(Forty years later he's not done yet, but it doesn't matter. Not as long as one of them is still breathing.)