What was the name of Einstein's sister?

Einstein’s sister was Maria “Maja” Einstein, later known as Maja Einstein Winteler.

Born in Munich in 1881, Maja was Albert Einstein’s younger sister and the only sibling mentioned in this question. The family later moved to Italy, and Maja studied Romance languages and literature in Berlin, Bern, and Paris. She earned a doctorate from the University of Bern in 1909.

Maja married Paul Winteler in 1910 and lived for many years in Switzerland and Italy. Albert supported the couple financially, including a large payment in 1924 to help settle debts on their estate near Florence. Her life was disrupted by the rise of fascism: she moved to the United States in 1939 and lived with Albert in Princeton.

A common mix-up is Elsa Einstein, who was Albert’s second wife and also his cousin—not his sister. Maja died in Princeton in 1951.