The abuse had begun when they were children. Sherlock was always the first in line to criticize his elder brother and regularly played pranks. Throughout his teenage years, Mycroft had often woken to find dead things in his bedroom, or beetles and ants in his bed. He hated it but he struggled to hide his reactions from his little brother. Sherlock thrived on Mycroft's reactions and his pranks only increased as they both grew up. Mycroft trained himself to be cold and indifferent towards the pranks that were regularly pulled on him and eventually Sherlock stopped. Filling Mycroft's shoes with maggots and putting dead mice in his desk drawers soon lost its appeal as Mycroft's emotions became hardened and controlled.
Sherlock stopped his pranks and instead turned his attention to using words as a weapon. He saw the wall that Mycroft was building to protect himself but he also saw how easily that wall could become cracked and broken. He studied Mycroft for weeks, watching how certain taunts affected him. A few harsh comments about Mycroft's weight and his brother spent almost a whole summer eating nothing but salad and fruit. Mycroft was Sherlock's private project, the one human being that he could study every day. Mycroft was a psychologist's dream case study; with a mind so intelligent and above average but with self-esteem so low it restricted him considerably.
Mycroft had no control over the impact Sherlock's words had on him, but the one thing he did have control over was his weight and what he ate. He strived to be slimmer and fitter, thinking the taunts would stop once Sherlock saw that he was no longer fat. Mycroft worked hard, counting calories and constantly weighing himself to make sure that his weight was decreasing.
"Mycroft, this isn't healthy. You're going to do damage to yourself. You need to eat meat or at least some cheese, you'll become ill without protein." Mummy said with worry as Mycroft weighed his salad before he ate it.
"I am perfectly healthy, Mummy. I want to lead an active, fat-free lifestyle." Mycroft replied, taking no notice of her words.
After almost a year of Mycroft starving himself to become slim, Mummy had enough. She took her teenage son to a doctor who arranged counseling to begin the following week.
"So what's wrong with you? Bulimia? Anorexia? You're such a freak, Mycroft." Sherlock taunted as soon as Mummy left the two of them alone, "Look at you. You're so ugly. You can't even diet without getting it wrong!"
Mycroft just nodded, unable to even think of a reply to his brother. He remained thin and trapped by his cruel younger brother until he was able to move out and go to university. Living away from home gave him freedom to live his life without taunts. He gained weight rapidly, indulging in junk food and cakes in a way he never had previously. It felt amazing to eat meat, cheese and sugar again after years of just salad and water. The flavours tasted wonderful and he was able to ignore his expanding waistline as he focused on his studies.
After three years of indulgence, Mycroft returned home with a 1st degree from Oxford and a large waistline. Sherlock had grown up and matured but that didn't mean that he'd changed. He had three years worth of insults and taunts ready and waiting in his mind palace. The constant psychological abuse sent Mycroft back into a downward spiral of weight loss and caused him to flee from home and to London instead.
Just a few years later and it was Sherlock who was suffering. He'd gotten himself into a bad circle of friends whilst in his first year at Cambridge and had been kicked out for drug use. Sherlock went from place to place, begging and stealing to fund his drug addiction. It was Mycroft, the brother he'd bullied since childhood, that came to his aide.
"What are you doing here? I don't want you to be here." Sherlock slurred his words, clearly high, as Mycroft approached.
"Cambridge called to say you'd been kicked out." Mycroft said, looking around the dirty flat that Sherlock was currently living in, "Goodness knows why you prefer this to your room at the university."
"Come to give me a lecture?" Sherlock hissed.
"Of course not, that would be a complete waste of my time." Mycroft replied, "I just wanted to check that you were still alive, brother."
"Get lost, Mycroft." Sherlock muttered.
"Gladly. Try not to overdose, brother mine." Mycroft said, turning and leaving the flat quickly. He disliked seeing Sherlock high as he cared for his younger brother immensely. He visited Sherlock twice a month at first, but ended up dragging his brother out of drug dens twice a week. Sherlock descended rapidly into drug abuse; he barely even recognized his own brother as he was dragged out of drug dens in the early hours of the morning. Mycroft finally had enough and had Sherlock put away into a rehab clinic. He enjoyed being the one in control for once, as Sherlock had manipulated and controlled him when he was younger.
"You're enjoying this." Sherlock accused on one of Mycroft's visits.
"Of course not, brother mine." Mycroft replied with a slight smirk.