[The joke flies way over Sherlock's head, because to him, the answer is quite obvious (and had better not be jossed by DGS2).]

I do have a brother. However, he's utterly irrelevant to the situation, as Mycroft would never be able to pass himself off as the World's Greatest Detective. There is only one Sherlock Holmes and it is the man crouching before you! [He strikes a flamboyant pose and a few sparkles shimmer in the air surrounding him.]