Every once in a while, when Sherlock is cross with Mycroft, he’ll let himself into Mycroft’s house at night and, in the dark, skew the furniture ever so slightly that the average person would never notice. But Sherlock knows it drives Mycroft, and his OCD tendencies, up a wall.
One night, Sherlock has just moved the side table in the living room when he hears the door start to open. He thought Mycroft would be gone longer. Sherlock hides himself in the adjacent hallway as light creeps its way in. Mycroft isn’t alone; Sherlock can hear two sets of footsteps stumbling towards him. As the sounds round the corner into the living room, Sherlock can see the pair of back-lit figures are kissing, but what surprises him is the figure beside his brother. Greg Lestrade. Sherlock’s mind comes to a screeching halt. How could Lestrade possibly be interested in Mycroft? Sherlock never thought the crush he had for the silver-haired inspector would come to fruition, but surely he was a better choice than Mycroft. Sherlock was so lost in thought he hadn’t realized he’d let out a small noise of discontent until Greg pulled away from Mycroft.
“Did you say something?” Greg asks Mycroft.
“My mouth was occupied with yours.”
“Must be hearing things.” Greg smiled.
“Is there something you would like to hear?” Mycroft asks leaning in.
Greg pauses, “Music.”
“Music?” Mycroft questions leaning away.
“Yeah.” Greg says fishing his phone out of his pocket, “Dance with me.”
Soft music begins to fill the room from speakers in the nearby shelves as Greg takes Mycroft’s hand and waist and starts to gently sway.

If asked, neither Mycroft nor Greg could tell you the moment they fell in love. But Sherlock knows. He saw it happen in front of him. He watched the pair dance and laugh and kiss. And he saw a side of his brother he never even thought existed. As he waited for the two to ascend the stairs so he could make his escape, Sherlock knew that he was never the Holmes brother Greg would choose.