Della Duck

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 also the mother of Huey, Dewey and Louie, Donald's triplet nephews.

Background
Donald's sister was first mentioned in the cartoon short Donald's Nephews (1938), in which she sent her sons Huey, Dewey, and Louie to visit Donald.

The nephews' mother made her first visual appearance in Mark Worden's illustrated version of Carl Barks' Duck family tree, by the name Thelma Duck, which was given to her by Barks. However, in Don Rosa's Duck Family Tree, first published in 1993, she was given the first name Della again. Della later appeared as a child alongside her brother and her parents in Rosa's The Empire-Builder from Calisota, chapter 11 of the Eiser Award-winning comic book series The Life and Times of Scrooge McDuck. According to this comic, Della and Donald are twins.

The relationship with her sons and Donald, and why she leaves them with her brother, is debatable. According to the Dutch Disney comic 80 is prachtig, Della was one of the first female pilots, and eventually became an astronaut, leaving her children with her brother just before she left for a space trip. This comic was made in honor of Donald Duck's 80th anniversary in 2014, and was published in a special issue of the Dutch Donald Duck magazine.

In the newspaper comic strip of November 21, 1937, Donald, who has grown increasingly tired of his nephews, receives a telegram from Della in which she says that she wants Huey, Dewey, and Louie to come back home, which they do, much to Donald's pleasure. However, the trio returned to appear in the comic strip not long after their departure. As of the 1942 cartoon The New Spirit, they were listed as being adopted by Donald. This is also carried into the 2017 version of DuckTales, where it is shown that Donald raised the boys on his own, with Della mysteriously missing.

DuckTales (2017)
The mystery of Della's absence in the nephews' lives is a central story thread in the first season of the show, with the ending of the series premiere having Dewey discover a painting, showing that she was previously a companion of Scrooge and Donald's in their adventures. Unlike most portrayals of the character, her hair color is now white as opposed to blonde.

In "The Great Dime Chase!", Dewey goes to Webby Vanderquack for information about Della, mentioning that Donald had never said more than one word about her and their only memento of her is a photo of Della pushing Donald's face into a cake. Sadly, Webby has little to no information on Della, as if she disappeared off the face of the earth. She mentions that Scrooge bought a mail company the day after they delivered a piece of junk mail turned up at Della's door. Thus, Dewey and Webby go to the archives at The Money Bin but Emily Quackfaster kept stalling them with so-called trials. Eventually, Dewey finds a book titled with his mother's name with the only entry being a reference number, which led him and Webby to a secret room, where they find a letter from Della to Scrooge, revealing she had stolen the Spear of Selene, which was mentioned in "Woo-oo!". Fearing the possibility that his mother betrayed Scrooge, Dewey tells Webby to hold off telling anyone until they know more.

Classic comics
Although Della never appeared in the classic cartoons, she was mentioned on rare occasions throughout Disney's published comics.

A comic adaptation of the short Donald's Nephews was written into the Donald Duck newspaper comic strip of 1937, in which it was told that the nephews' father was in the hospital after a giant firecracker had exploded under his chair. In this comic, Della was still said to be Donald's cousin, rather than his sister.

The nephews briefly mentioned both of their parents while saying their prayers in a 1960 installment of the Donald Duck newspaper comic strip written by Bob Karp and drawn by Al Taliaferro.

DuckTales (2017)
Della is confirmed to appear in the comic adaptation for the 2017 series. The solicitation for Issue #2 of the tie-in comic reveals that one of the stories featured in that issue will expand on her adventures with Donald and Scrooge.

Trivia

 * "Della" was also the first name of Al Taliaferro's aunt.
 * In an early French translation of the comic adaptation of Donald's Nephews, Huey, Dewey, and Louie were sent to Donald by Clarabelle Cow instead of his sister.
 * Della made a very obscure cameo appearance in Don Rosa's The Sign of the Triple Distelfink. During a flashback of Gladstone, she can be seen at young Gladstone's birthday party.
 * Della's introduction was the first time a Sensational Six member was explicitly said to have a sibling (Morty and Ferdie's mother was already introduced in 1932, but she was said to be Mickey's sister years after her first appearance).
 * In The Duck Who Never Was by Don Rosa, it's shown that if Donald wasn't born, Della would have left her children with her cousin Gladstone.
 * In DuckTales (2017), Della is shown wearing a pilot's outfit, perhaps as a parallel to her brother's sailor attire and/or a nod to her comics occupation.