All three remaining members of the Holmes family are invited to the Royal Wedding.
Sherlock goes only because Mummy insists, and because she wants them to arrive together, he spends the night with Mycroft and Mummy in the family townhouse in Mayfair. Mummy had told them to be ready by half past eight, yet Mycroft finds Sherlock still fussing with his attire at a quarter past.
This is not the first time they've had to wear morning dress, nor will it be the last, but Sherlock does have less occasion for dressing so, and it shows. Mycroft watches Sherlock fuss with his ascot for another few moments before going over to help.
Sherlock lets him. Sherlock lets Mycroft tie his ascot into a Cocolupa knot (Mycroft doesn't bother to ask because he already knows), and the royal purple silk is a delectable contrast against the Tyrian purple of Sherlock's waistcoast. Sherlock may profess to care little about mundane things like his dress, but he has always known which colours flatter him best. Mycroft has few weaknesses, and Sherlock doesn't hesitate to use those few he has against him.
It takes little time afterward for Sherlock to button his waistcoast and morning coat, and he lets Mycroft fold his pocket square (same royal purple as his ascot and ironed vigorously the day before) into the three stairs style before tucking it into his breast pocket. Mycroft steps back and looks at him, and Sherlock is utterly striking, the contrarian in him still peeking through the strict formality surrounding him. Mycroft captures the moment in his mind.
Sherlock smiles at him, sharp and knowing. "What will be my reward if I behave?" he asks, and Mycroft responds with a slow, satisfied smile.
Mummy fusses over Sherlock's less-than-perfectly respectable choices in colour when they join her in the hall. In defence of his brother, Mycroft assures her that Sherlock's apparel will appear perfectly smart in comparison to what the majority of the other guests will be wearing. He is correct, of course, and Mummy fusses over the the general lack of ascots ("Even the Prime Minister, Mycroft!") and the audacity of the Prime Minister's wife in going without a hat or even a fascinator.
Sherlock smiles in satisfaction, puts on an affronted tone and says, "Mummy, you should place more trust in me. I wouldn't have done anything scandalous for an occasion such as this." Mummy pats his hand, half in apology and half in pride, and Mycroft's eyes meet Sherlock's over her head.
Sherlock tilts his head, his smile turning into one of challenge, and Mycroft tips his hat in subtle acknowledgement.
He keeps the pocket square, later. They have no need to spare any expense, anyway.