The Wondersmith
Lady Astoria Holmes had always wanted two sons. One for her husband, to be trained as the heir—the perfect little politician—and one for her. When Mycroft was four, the doctors told her that she couldn't have another child. But a year later, when her twin sister tried to smother her own newborn son, Astoria found herself to be the mother of two children. Astoria was very fond of unusual names, but Cecilia had already named the child Sebastian. Mycroft, like his father, had thick dark hair. Sebastian on the other hand followed after his mother and aunt with his soft, strawberry blond hair. Astoria decided to never tell Sebastian that she wasn't his biological mother, as his mother was locked away in a mental institution.
Sebastian was Astoria's favorite child until Sherlock, the unexpected little miracle (as everybody but Sebastian saw him), came along when Sebastian was seven. After that, Sebastian was essentially forgotten. As the middle child, he was ignored by his father, who only cared about Mycroft, and his mother, who was obsessed with her precious little baby boy.
When Mycroft was thirteen, he started going to boarding school, because Father thought it would help him make connections, and learn how to interact with others his age. Even with Mycroft away, Father just stayed away for work longer, not bothering to spend much time at home now that the only child he cared about was away.
Sebastian, who never really cared enough to be smart academically, was sent to a local school. Of course this school was just as posh as the boarding school Mycroft attended, so Sebastian wasn't able to play normal sports like football and rugby. Instead, he found himself becoming the school's best fencer, archer, and hunter.
Sherlock was a brilliant little child, understanding calculus before he started speaking. When he finally decided to start talking at five, he spoke in full sentences and with a larger vocabulary than most grown men. When Sherlock was ten and Sebastian was seventeen, Sherlock became interested in genetics. Sherlock had always wondered why he had dark, curly hair, Mycroft had dark straight hair, and Sebastian had straight blondish hair. As his mother had bought him a room full of very expensive and complicated scientific equipment, Sherlock decided to secretly examine the hair-related genes of his family. He was fascinated by the results, and decided to share them with his family at dinner one day. As usual, Father was away on some diplomatic trip, so only Astoria, Sebastian, and Sherlock sat at the huge antique mahogany dining table.
"Mummy, guess what I figured out today?" Sherlock asked excitedly.
"I don't know, Sherlock, you have been quite busy in your laboratory lately," said Astoria adoringly.
"Well, since you're blonde, you have two recessive hair colour genes. Mycroft and I are heterozygous, and Father is homozygous dominant. That means that Seb shouldn't be able to be blond. I think he's a mutant," Sherlock giggled.
Astoria, usually quite pale, went as white as their expensive marble staircase. Neither Sherlock nor Sebastian seemed to notice this, however.
"Hey, shut it you little freak! If anyone's a mutant, it's you!" Sebastian shouted angrily.
"Sebastian, do not talk to your little brother like that. Apologize now."
"No, Mother, I will not. I'm sick of you always taking his side. The little freak just called me a mutant, and you're not going to do anything about it?" Sebastian stormed off to his room, slamming the door loudly.
Later that night, Astoria carefully entered Sebastian's room with a steaming mug of his favorite tea. Sebastian was still upset about the possibility of being a mutant and about his mother's obvious favoritism towards Sherlock.
"Sebastian, you're not really a mutant, don't worry. Well technically everyone has mutations, that's how genetic variation is created and…"
"Darling, I'm really sorry. I wanted to tell you, but I thought it was for the best…"
Sebastian stopped ignoring her and looked up curiously.
"Sebastian, love, I'm not really your mother."
Sebastian gasped, and looked at her intently, hurt.
"Your biological mother is my sister. She… couldn't take care of you. She's in a mental institution."
"And… my… my father? My real, biological father?"
"He died heroically in a war shortly before you were born. Your mother couldn't handle the stress and she thought you looked a lot like him."
"So you've been lying to me for my entire life?"
"Sebastian, it's not like that. I was trying to help—"
"Get out."
"Sebastian—"
"I said get out, Astoria."
Astoria felt hot tears streaking down her face, and fled.
After that incident, Sebastian essentially moved out of the Holmes Manor, only spending time there occasionally to sleep. He never said a word to Astoria again.
When Sebastian was eighteen, the day after he graduated, Astoria found a note on his pillow. "I'm off to join the army, like my REAL father," it said in his scrawled handwriting.
Authors note: So this is my first published fanfic… Please feel free to review with constructive criticisms. I don't have a beta, and I definitely don't own the characters. The title is from the song The Wondersmith And His Sons by Astronautilus, who looks kind of like my head-canon of Sebastian Moran. Please forgive me making up a name for Mummy Holmes, I thought she needed a name, and I kind of just randomly picked one that seemed like a plausible name for one of their type of people. I would like to continue this, but I'll see if there's even anyone who wants to read more. In the future (if I continue this story) I will go into Seb's time in the army, how Seb met Jim, how I see his relationships with Sherlock and John in the future, and eventually some Moriarty and Moran friendship/relationship… Thank you for reading this!