Introducing Mycroft Holmes
I have no idea why, but the question that I receive most often by e-mail is, "What was the name of Sherlock Holmes's brother?"
The answer: Mycroft.
Mycroft Holmes appears as a character in two of the Holmes stories: "The Greek Interpreter" and "The Bruce-Partington Plans." Holmes tells Watson that Mycroft is "seven years my senior" (older by seven years), and brilliant in observation and deduction, but so lazy—and fat, it turns out—that he seldom moves from his accustomed cycle: his rooms, his office in a government building, and the Diogenes Club.
In the earlier story, Holmes says vaguely that Mycroft "audits the books in some of the government departments." By the time of the later one, Watson finds out that in fact Mycroft has a position of importance and delicacy: "occasionally he is the British government. . . the most indispensable man in the country:
We will suppose that a minister needs information as to a point which involves the Navy, India, Canada and the bimetallic question; he could get his separate advices from various departments upon each, but only Mycroft can focus them all, and say offhand how each factor would affect the other.
Many Sherlockians have interpreted this passage, and the role played by Mycroft in bringing the Bruce-Partington scandal to his brother's attention, to mean that Mycroft was an early and important member of the British intelligence establishment.
Mycroft also plays a tiny role in "The Final Problem" and is mentioned in "The Empty House."
The rest of the family
It is natural to wonder whether there were other brothers in the Holmes family. None are mentioned in any of the original Sherlock Holmes tales. But the 1975 film "The Adventure of Sherlock Holmes' Smarter Brother." starring Gene Wilder, is not about Mycroft; instead, Wilder presents a manic third brother, Sigi Holmes. Presumably the name is short for Sigerson, taken from the alias used by Holmes in "The Empty House."
A number of Sherlockians have ventured to speculate about a brother named Sherrinford. That name is taken from Arthur Conan Doyle's original notes for A Study in Scarlet, in which the name Sherrinford is used for the consulting detective who would shortly appear in print under the name of Sherlock.
And what about sisters? There is no evidence, apart from a few wistful comments in "The Copper Beeches" to the effect that "no sister of mine" should run the risk that faced Violet Hunter.