Joss Whedon has confirmed the inclusion of Scarlet Witch and Quicksilver in Avengers 2 during an interview with IGN. When asked about the brother/sister duo, he said “You know, they had a rough beginning. They’re interesting to me because they sort of represent the part of the world that wouldn’t necessarily agree with The Avengers. So they’re not there to make things easier. I’m not putting any characters in the movie that will make things easier.”
Also, in the same interview he talked about the ending to Iron Man 3 and what that means for Tony Stark’s involvement in Avengers 2.
*Iron Man 3 spoilers: Highlight below to read*
“Well, I feel like in Iron Man 3, even though he said, ‘I’ve changed’ — he blew up his remote suits, but I don’t think anybody thinks he doesn’t have one anymore. The question is, if The Avengers are called, does he show up? And the answer is, ‘Yes!’”
Take from that what you will.