Growing up Holmes must have been a unique experience, if Sherlock and Mycroft's interactions are anything to go by.
Here are three takes on the possibilities:
Sonata Form by
cj_ludd18 A fic full of vivid details. Sherlock and Mycroft, through Mycroft's eyes.
Stradivarius and Grace by
snowlight Subtle and gorgeous. Sherlock and Mycroft, and the story of Sherlock's violin.
Bonheurs Enfantins, Trop Vite Oubliés Effacés by
fyria_yiries Every now and then, the British government sat down and folded paper. A history of Mycroft and Sherlock that'll make you feel sorry for them both.