What is the surname of Ezio's sister in Assassin's Creed II?

Ezio's sister in Assassin's Creed II has the surname Auditore.

Claudia Auditore is one of the members of Ezio's immediate family. She is introduced alongside Ezio, their brother Federico, their younger brother Petruccio, and their parents, Giovanni and Maria Auditore. The family's position in Florence changes dramatically after Giovanni is accused of treason.

Claudia's story continues beyond the family tragedy. In Assassin's Creed: Brotherhood, she becomes involved in managing the Assassin property at Monteriggioni and later trains as an Assassin. This makes her more than a background character in Ezio's trilogy.

Auditore is Ezio's family name, not the name of the Assassin organization. The organization is generally called the Assassin Brotherhood, while the rival faction is the Templar Order. Those terms are often mixed up when discussing the series' characters.