Warning: The following is very short and very sweet; it'll rot your teeth with the sweetness of it. Also, note that these short drabbles won't always go in order. Just whatever idea strikes me is what I'll write.
The day young Sherlock was born, Mummy got very sick and Father went far away. Mummy sat in her chambers and cried while the maids tut-tutted around her and Sherlock. There were mumbles about affairs and lovers when they thought no one was around to hear. Mycroft, seven and very good at hide and seek(particularly the hiding part), was always there to hear though. It was at seven that he placed himself in the position of man of the house.
He came in every morning to sit with Mummy and to stroke Sherlock's downy black hair. Then he'd leave for a day of teachers and books and politics(he did so love them). Once finished, he'd come back at precisely four in the afternoon. He'd settle on the bed with Mummy, tucked under her arm with his head resting against her bussom. She'd stroke his ginger hair and let him hold Sherlock safe and close and warm. Sometimes, when she wasn't feeling particularly ill, she'd read Frankenstien outloud. Sometimes, when she was feeling particularly ill, Mycroft would read Frankenstien outloud.
The days and weeks pass slowly and Sherlock gets bigger and bigger while Mummy gets better and better. Mycroft keeps his constant vigilant, visiting every morning and staying every afternoon. He learns how to bathe Sherlock and how to feed him. The maids titter around him, cooing at him. "What a good big brother you are!"they'd say.
Mummy finally goes back to work when Sherlock is two months old. She isn't there when Sherlock says his first word at three months(which, was quite clearly, Mycroft). She isn't there when Sherlock finally becomes mobile(crawling toward Mycroft at six months and again, tottering around the manor behind Mycroft at eight months). She isn't there when Sherlock begins to read from his very first book(at a year old and from one of Mycroft's old chemistry text books). Indeed, she isn't there for a lot of Sherlock's firsts...
... but that was okay; Mycroft was.