What is the name of Sherlock and Mycroft's sister, revealed in Season 4?

Sherlock and Mycroft Holmes’s sister, revealed in season four, is Eurus Holmes. She is introduced as a hidden member of the Holmes family and becomes the central antagonist of the fourth series.

Eurus is first foreshadowed in “His Last Vow,” when Mycroft hints that Sherlock has another sibling. Her identity is revealed near the end of “The Lying Detective,” after she has appeared under several disguises, including as John Watson’s therapist. This disguise-driven introduction is why viewers can miss her earlier appearances.

In “The Final Problem,” the series establishes that Eurus is the youngest Holmes child and possesses extraordinary intellectual ability, even surpassing Sherlock and Mycroft in certain respects. Her intelligence is paired with a profound inability to understand ordinary human emotion, making her dangerous as well as brilliant.

Eurus is played by Siân Brooke, while Indica Watson portrays her as a child. She is not Irene Adler, Sherlock’s adversary and romantic fascination, and the names Violet and Sarah are not alternate identities for the Holmes sibling.