"Is that the Scarlet Witch? No, that's his little sister," Singer said. "I even had a line which I cut, where Quicksilver's mother says to the little girl, 'Go up and bug your sister,' and the little girl says, 'She bugs me!' You never see the older sister, but it was to imply that there is an older sister for comic bool fans. I ended up cutting it."
In the comics, Quicksilver and Scarlet Witch are twins -- the twins, in the forthcoming Avengers: Age of Ultron, as teased at the end of Captain America: The Winter Soldier. Singer -- who has since pulled out of press for the movie, so this may be one of his final interviews on the subject -- expressed that he would like to do more with Quicksilver in the next film.
"Even though our Quicksilver is not a main character like in The Avengers, it's a character there's a lot of thought and concern about and it's a character I'd like to explore further in the sequel," the director said.
In the comics, Scarlet Witch and Quicksilver are longtime members of The Avengers, but their father is the X-Men villain Magneto. As such, Marvel and Fox share the film rights to the characters, with restrictions on each as to exactly what can be said about the characters' backgrounds.
Magneto does have three other daughters in the comics -- or at least two, and a third "presumed." That would be Lorna Dane, better known as the X-Factor member Polaris; Anya Eisenhardt, a powerless daughter who died as a result of an anti-mutant hate crime; and Zaladane, former leader of the Savage Land Mutates, who claimed to be Lorna's sister.
Given Quicksilver's age relative to Magneto's, though, it isn't entirely clear that they will use that family connection in the films (which would be ironic since it's one of the handful of things that they have which Marvel Studios cannot use).