A.N. I have gone through this story in meticulous detail, adding scenes and taking out bits that don't work any more. I really recommend starting reading again. You don't have to, but there are lodes of really nice scenes with John and Sherlock, Lizzie and Mycroft or even Harry and Mycroft. It's also about 4000 words longer. So yeah, Ono obligation, but I think you should start again!
"Why did they give us up?" the little boy pulled at his brother's jacket.
"Why should I know any better than you, Lock?" Mycroft said gently, with a hint of frustration. His brother was always annoying, but Mycroft knew the boy had had to suffer for it with his father. So he tried never to show how angry he made him.
"You know everything" he said, in awe of the fountain of knowledge that was his big brother.
"No I don't"
"You knew when Daddy was drunk. I didn't know"
"I told you how to know, though, didn't I? Pupils dilated, slurred speech, clumsiness, anger, smell"
"I know. Why was Daddy cross with me?"
"He was drunk, Lock. He didn't mean it"
"But it hurt" Mycroft looked down at the child, who was looking at him from under his brown curls. He had a large bruise covering his cheek and eye.
"I know. You've got to be strong, now, Lock"
"Do we have to live with strangers?" He looked down at his trainers, scuffing the floor.
"Yeah. Yeah we do. But they'll be nicer than Daddy, you just wait and see"
"But you promised you wouldn't let Daddy hit me like he hit you" Sherlock looked frightened. He had always trusted his big brother. Mycroft never broke his promises.
"I've always managed to stop him before"
"Not other than tonight? He's never hit you before?" Mycroft asked, his heart heavy with dread.
"Yes he has"
"When, Sherlock? Why didn't you tell me?"
"Last week he hit me with his belt. Only once, not like you. It really hurt"
"Why didn't you tell me?"
"He made me promise"
"Has he ever asked you to keep anything else a secret, Lock? Anything at all?" Mycroft knelt down, gripping his brother's shoulders, shaking him slightly, desperate to know.
"Only when he makes me watch you get hurt, and he tells me not to tell anyone. Let me go, Mycroft" Mycroft stood back up, dusting his trousers. Sherlock slipped his hand into his brother's.
"You did a good job, Lock. I promise"
"Why did Daddy want to hurt you?"
"I did bad things, Lock. I disobeyed and did things he didn't want me to do"
"What kinds of things?"
"I don't know. I bought you that toy car, remember? And I was late home from school"
"But you were screaming, last night"
"I know" he sounded conflicted, as though he wanted to both protect his brother from the truth and at the same time hide his terror at the attacks.
"Did it really hurt?"
"Yes, Sherlock, it did. But the doctor mended my arm, see, I've got a lovely red cast" Mycroft held out his arm so the five year old could see the cast. "And he mended my ribs, and my foot, and stitched up my back. I'll be fine"
"Do I have to have a cast?"
"No Lock. You're not badly hurt at all, are you"
"Good boy" he smiled.
"Will they be nice to me?"
"Yes, of course they will, you're cute, they'll love you"
"You're not cute"
"Very astute of you Sherlock" Mycroft said sarcastically
"But you're not. Does that mean they won't love you?" Sherlock asked. Mycroft bit his lip. It had been worrying him.
"I don't know"
"Did Daddy love us?"
"Don't parents love their children?"
"Yes, normally they do. But Daddy wasn't capable of love, Lock. He was too in love with work and alcohol"
"Why didn't he love us?"
"Because he didn't want us. We got in the way"
"In the way of what?"
"His whores" Mycroft whispered, too quietly for the little boy to hear him "In the way of his life, Lock. He wanted a life with no responsibility. Children don't fit into that"
"But why did he hit you so much?"
"Because he couldn't hit you"
"You're too little"
"But why did he hit you?"
"I annoyed him. He needed someone to blame when things went wrong. Stress release"
"Why did you let him hit you?"
"I didn't let him! He was bigger than me"
"You're bigger than me, and when you hit me, I bite. Why didn't you just bite Daddy?"
"Because, Sherlock, when someone comes at you with a belt and a bottle of vodka, you don't bite them, you stand still and take it like a man!" Mycroft snapped. Sherlock drew his hand away from his brother's and stared at the ground. He detested shouting. "Sorry, Lock, I've had a bad day"
"Yeah. Bad day"