Mycroft watches as his little brother begins to explore his surroundings, as well as his own abilities. Soon, he is walking, talking, (and complaining) like his older brother.
Story
It had been a month since Mother had come home from the hospital, Sherlock in tow, and Mycroft was getting quite bored and impatient with Sherlock's growth. All he wanted was to have a sibling to play with, but all his brother had done so far was gurgle, sleep, cry, eat, grab for toys he couldn't quite reach, and sleep some more.
Because of this, Mycroft had decided that babies were boring.
Until Sherlock sat up for the first time, blinking and gurgling happily- the most interesting thing he had done in the twenty-odd weeks he had been at home. A few days after that Sherlock had successfully grabbed the toy Mycroft had held out to him.
Two months later and Sherlock was just getting the hang of crawling around. Mycroft soon realized that babies were slow and took a lot of patience. Patience that Mycroft didn't possess, even at 7 and a half years old. He also realized that once Sherlock started, he wouldn't ever stop or go slower than he was already going. The elder compared his brother's growth rate to a linear function his father had shown him once.
A constant rate of change with no stopping or slowing down.
As Sherlock's growth increased, however, Mycroft began to compare his brother's growth to an exponential function. Once he started, he never stopped, skyrocketing off the graph.
"Mikagoga!" Sherlock babbled. Mycroft shook his head. "No, Sherlock. It's Mycroft. My. Croft."
"Myco!" Sherlock exclaimed.
Mycroft sighed. Ever since Sherlock had begun talking about a week after Mycroft's 8th birthday (his first word was 'bee', an insect Sherlock seemed to be quite fond of. He always chose yellow toys, he would only sleep with his bumblebee plushie, and he fussed whenever he was dressed in something other than one of his many bumblebee themed onesies), Mycroft had been trying to get Sherlock to say his name.
He hadn't gotten very far- the most Sherlock could say was 'Myco'.
Better luck tomorrow.
Tomorrow was, predictably, the exact same. In talking, at least. All Sherlock was saying was 'Myco', 'mummy', 'bee', and 'window', a strange new word he had picked up.
In terms of other development, there had definitely been some growth. Mycroft had been sitting in a plush leather chair, legs crossed, reading a book on the solar system he had stolen from his father the week before. Sherlock was playing happily a few feet away, sprawled out on a dark, forest green blanket.
It was silent, aside from Sherlock's giggles and happy babbling, broken by the occasional 'window'. Mycroft knew that Sherlock would grow up to be someone who constantly talked and rambled on about things he would grow to be passionate about.
The next thing Mycroft knew, Sherlock was hitting his ankles, cooing softly.
Mycroft put down his book and leaned forward, staring into Sherlock's bright blue eyes. Sherlock babbled happily and Mycroft's forehead creased with confusion.
Wasn't Sherlock three feet away a few seconds ago? How could he have gotten from point A to point B by himself? Mycroft had thought.
His questions were answered, however, when Sherlock turned away and reached up, grabbing onto the edge of Mycroft's chair. In no time at all, Sherlock was walking with the help of Mycroft's chair. When he reached the corner, he dropped to his hands and knees and began crawling back to his blanket.
Mycroft watched in awe as Sherlock crawled away, cooing and blowing raspberries on his journey. His chest swelled with pride and Mycroft grinned. He usually only felt pride when he won a prestigious award at his school, but now he had another reason to feel this positive emotion.
He had seen his baby brother crawl before, but he had never fully appreciated what he was doing before then. But now he knew: his baby brother was moving!
The next week brought on plenty more surprises, including Sherlock taking his first steps.
He had been "walking" with mother, i.e., holding her hands and shuffling forward, when Mycroft had entered the room to say hi to his mother and brother at the end of his first day of fourth grade.
Sherlock had seen him, smiled as widely as his baby cheeks would let him, and yelled "MYCO!"
Mycroft froze, never experiencing this kind of situation with his brother. Sherlock squealed and let go of their mother's hands, much to her surprise.
Sherlock took a step. Then two steps, three steps, five steps, until he promptly fell down. He blinked, frowned, and dropped himself into his crawling pose.
Sherlock pushed himself upright into a standing position and took another step before he fell again, this time forward. Sherlock sniffed a few times and started to wail. "Myco! Myco!" he shouted between sobs.
"Oh, Sherly, come here." Mycroft cooed, leaning down and picking his brother up, cradling the infant in his arms like he had all those months ago.
Sherlock's cries stopped almost instantly and he gave a watery smile. "Myco!" He cooed.
Mycroft smiled right back. Sure, babies could be boring, but the results were worth it. As Mycroft stood there, Sherlock in his arms, he reflected on the past few months and realized that the highlights of said months had been Sherlock's series of little firsts, including walking and talking.
Mycroft sighed and looked down at his gurgling little brother. "I adore you, my little baby brother mine."