Martin had never really understood his step-brothers. Mycroft mostly just ignored him, which was to be expected – the elder Holmes was sixteen and Martin was only seven, so they didn’t really have a whole lot in common. Besides, Mycroft was away most of the time - Martin wasn’t really sure where he went - so they didn’t see much of each other.
Sherlock, though… Sherlock confused him. He was only two years older than Martin but whenever anybody - especially Mycroft – was around, Sherlock treated the younger boy like he was nothing. He sneered at him and called him an idiot, wearing that cold mask of indifference he put on for everyone. At dinner with their parents he went out of his way to ridicule anything and everything Martin said.
But when they were alone, Sherlock was a completely different person. He would run through the fields with Martin while they pretended they were aeroplanes. They’d have sword fights with sticks they’d found in the woods, re-enacting the pirate battles they’d seen in the movies. They’d skip rocks, chase rabbits, and catch butterflies, then flop down in a field to stare at the clouds as they went by. Sherlock would pace back and forth in the fort they’d made from blankets and low-hanging branches as he told Martin everything he’d learned from his experiments the day before. Martin always stared in open-mouthed wonder at the things Sherlock knew, and Sherlock grinned at his adoration.
But the instant anyone else was around, the mask snapped back into place.