Entertainment Weekly confirmed speculation based off a Joss Whedon tease from last week. Their Iron Man 3 cover story backs up a tip from an unnamed source with a quote from Marvel chief Kevin Feige.
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."

The draft could change, of course, but it's an intriguing update. What exactly does Whedon have in store for The Avengers 2? We're going to have to sit through four or five post-credit sequences to find out.