Winter is Coming

Sherlock and Mycroft might be unused to emotions and have very few weaknesses to be exploited. However, one weakness that terrifies both is their younger sister, who was born when Sherlock was seven. She has all of Mycroft's intelligence, and is as acrobatic as Sherlock. She was stolen, and trained to be an assassin. Her next hit is in 221B, and she doesn't know who her kill is.