Sherlock's mind is working a little bit better now and he runs rapidly backwards to when Mycroft was this man here, gentle and unassuming and ready to do anything for anyone, especially Sherlock. Before the government takes him and wrecks him and empties him.
He must be about forty. Yes. And he's here. Which means there's a chance he can see his brother happy. Oh, God, how Sherlock missed seeing his brother happy. Of course there's always the knitted concern when he sees Sherlock, as he works out the ins and outs of his brother in the time between them seeing each other, but then Mycroft smiles, jokes, shows the personality that Sherlock lacks but that his big brother more than makes up for and lets Sherlock bask in.
Sherlock reaches forwards for his tea, sips it, and it's perfect, of course. "Thank you," he says. "It's good to see you." It really is. "I've missed you." He really has, but it's not something he's ever said a lot.