A Stitch in Time was a digital comic strip published on Comic Creator, an app that allowed users to create their own comic strip stories featuring various Doctors, companions and villains.
The story's first "episode" was released for free along with the download of the app, and the further episodes were released upon the purchase of the app's packs. A total of 8 episodes were uploaded during 2016.
Ethel and Gretel, as well as their unseen sister, are all an allusion to the witches of The Wizard of Oz: Ethel represents the Wicked Witch of the West, Gretel represents the Good Witch of the North, and the third sister, who is said to have died after being crushed by the Doctor's TARDIS, represents the Wicked Witch of the East, who died after Dorothy's house landed on her. The final page of the story also see's Clara quote the film's famous ending line, "There's no place like home".
Because of the ubiquity of this race, no attempt will be made here to list every appearance in every medium. Instead, we focus on comic appearances here, and invite you to explore a complete list of appearances elsewhere.