What is the name of the Disney character who is Donald Duck's twin sister?

Della Duck, occasionally referred to as Dumbella Duck or Thelma Duck, is Donald Duck's twin sister and the daughter of Hortense McDuck and Quackmore Duck. She is the mother of Huey, Dewey, and Louie, Donald's triplet nephews.

Della Duck is first introduced to others as Donald's cousin; but, later it is said that she is Donald's twin sister. Della has her name first mentioned in a 1937 Donald Duck Sunday comic strip. At that time, she writes a letter to Donald, telling him that she is sending her sons to stay with him.

She had previously appeared in a comic story as a child in The Life and Times of Scrooge McDuck in which she and Donald are wearing identical sailor suits. She and Donald had been linked to Scrooge McDuck in equal measure. However, Donald was always referred to as Scrooge's closest living relative, suggesting that Della had disappeared or died.

Subsequently, a story told during Donald Duck's 80th birthday holds that Della was an astronaut and gave her nephews to Donald before she left for a dangerous space expedition.