As part of the new Iron Man 3 cover feature on Entertainment Weekly, Marvel Studios President Kevin Feige seemed to confirm that, at this stage in the writing at least, Marvel Studios plan to include Quicksilver and the Scarlet Witch in 2015's as-yet-untitled Avengers sequel. Here's the pertinent piece of the story:
A source confirms to EW that the twin characters Quicksilver and Scarlet Witch will be joining The Avengers 2, which Whedon hinted at recently when he said he was writing a brother-sister duo into the movie. Feige maintains a poker face on that scoop, saying only: "I'm not confirming or denying. The draft could change six months from now."
While certainly he didn't say that they're in the existing draft, it seems likely from the way he phrased it, coupled with Whedon's remarks last week, that the pair will join the cast of the anticipated sequel. The characters, the children of X-Men archenemy Magneto, have long-standing ties to The Avengers in the comics. As a result, ComicBook.com has speculated that they could actually be used in both the 20th Century Fox and Marvel Studios films to establish a shared Marvel Cinematic Universe that transcends licensing agreements. "We both have them," Feige told Superhero Hype last summer. "There's a specific arrangement with those two characters that would allow us to use them with Avengers, but not discuss or reference their mutant or Magneto-related lineage. They can use them as mutants and as Magneto's relatives, but cannot have anything to do with The Avengers." As there's been interest expressed by all parties in creating a shared universe, we speculated that if the studios were to cast the same actors to play the twins in both the Fox and Disney films that it might create a bridge between the movies, even if the actors in question are expressly forbidden from sharing their adventures with their teammates.