Author's Note: I don't know why I suddenly had the urge to write this, but I did. Warnings for mentions of upsetting themes. Oh, and reviews, as they say...are love.

Disclaimer: Sherlock belongs to Arthur Conan Doyle, and he's being lent to Moffie and Gatiss. In no capacity is he mine.

It happened quickly, only two weeks between the first cough and the final breath. Sherlock was only eighteen months old, and the only evidence he had for the case was a few old, blurred photographs and the briefest of recollections.

Mycroft was nine. He remembered it all.

They had always been sickly, frail with milky-white skin. Far too thin for children of that age - should have been pudgy and rosy-cheeked, but they'd never really caught up with babies their age after their birth seven weeks prematurely. Common in twins, the doctors said, not necessarily something to worry about.

Mummy did worry, though. Always made sure they were bundled up warmly on the rare occasions she allowed them outside at all. Fed them on a painstakingly healthy diet once they were weaned, kept everything as clean as could be.

Mycroft was more or less left to his own devices. He studied diligently, his evenings spent in the library rather than in the nursery. He knew he should be above petty resentment but he was, after all, only a little boy, and the twins dominated every minute of their mother's time, while their father was distant when he wasn't absent altogether.

It was cold that winter. Mycroft relished his walks through the heavy snow, leaving a perfect set of footprints behind him as he trudged through the grounds. He was not one to run and jump about but he liked the crisp, crunching sound his feet made as they cut through the blanket.

It was just a stray infection, Doctor Atwood said. It couldn't have been anticipated or prevented. No, it had nothing to do with Mycroft's having left the door open, and besides, the twins would be getting better within a week.

They didn't.

Mummy was inconsolable. Mycroft sat in the library on the bench-seat by the window, looking out at the expanse of snow and hearing her sobbing noisily from the next room.

By the ninth day one of the twins was at least showing signs of improvement. Mycroft supposed that as twins were the same, his sister would soon be catching up.

By the eleventh day Sherlock was stable, weak but out of immediate danger. Isabel was only getting worse.

Mycroft went to bed on the thirteenth evening without kissing the twins good-night as he usually did. It scared him to go into the nursery.

By morning it was all over. He knew it when he went down to breakfast and saw Mummy's eyes.

Mycroft didn't like to think it, but he couldn't help what he observed. Mummy wished it had been Sherlock.

She would never have said as much, of course. But she had always said before the twins were born how she longed for a daughter, and had been overjoyed when Isabel had arrived. And now she looked at Sherlock in a way Mycroft couldn't help but notice. She wished it had been him.

It was hard to explain to Sherlock, who, by four, was unmistakably a cynic. He could remember barely anything about her, and Mycroft was convinced his little brother thought of 'Isabel' as a fictional construct the family were all playing along with.

Sherlock was still a sickly-looking child, but the dark hair which curled over his ears now made him look adorable on an almost repulsive level. Mycroft saw Mummy looking at him sometimes and he knew she was imagining what Isabel would look like by this age.

"Tell me the story, Ay-coff," the little boy would plead on occasion. He was by far old enough to correct this pronunciation by now but clung to the original attempt with a stubbornness which was becoming characteristic. "The story about Is'bel."

And no matter how many times Mycroft would explain that it wasn't a story, that Isabel was - had been - real, Sherlock would refuse to believe it. Mycroft envied him that ability, and part of him wished they could just let Sherlock alone and tell him properly when he was old enough to understand. Mummy, however, insisted that it would be easier for him to grow up with the knowledge, so he needn't have an unpleasant shock later on.

Mummy no longer doted on Sherlock and did not go to any special measures to monitor his health as she had done before, despite the fact that he was still highly susceptible to illness. Not that she neglected him in any way, but Mycroft noted a sudden interest in himself and his studies which hadn't been there since before the twins' arrival. Gone was the mother who was always 'too busy' to come along to recitals and award presentations; these days she would even rearrange business meetings to come and see him play the piano in school concerts. When he was twelve she enrolled him in the top boarding school in the country and off he went, suitcase in hand and feeling positively valued.

Somewhere during his six-year absence, Sherlock isolated himself from his older brother. His letters, once rich with creative spelling mistakes, long, and highly amusing, gradually became cold, brief and free from error. Mycroft couldn't pinpoint the exact holiday when he really noticed that Sherlock had slipped away from him, but his visits became less refreshing and more and more tiring, so that eventually he was looking forward to going back to school from the moment he arrived home.

By the time Mycroft graduated - yes, first class, thank you - from university aged twenty-one Sherlock was almost unreachable. He was only fourteen but already pushing the boundaries of 'sullen teenager' into the practically sociopathic.

When Mycroft was twenty-five and working in maximum-security surveillance, Sherlock was setting his magnificent brain up for a slow rot, experimenting with cocaine and goodness knows what else but insisting he knew exactly what he was doing.

The first time he overdosed, one of Mycroft's men found him passed out in a churchyard. Mycroft never needed to ask whose grave he had been visiting.

The second time was on Sherlock's twenty-first birthday and at first Mycroft inwardly criticised his brother for such clich├ęd behaviour, until he remembered who else would have been twenty-one that day.

Mycroft doesn't leave doors open any more, regardless of temperature or weather. Sherlock still doesn't know why.