Speaking at the ABC Upfronts Joss Whedon has confirmed that the brother and sister team of Quicksilver and Scarlet Witch will be in Avengers 2.  Whedon’s comments indicate that the "brother-sister act" that he referred to as his "favorite characters from the comic book" in previous discussions about new characters who would be included in the Avengers sequel, are as previously reported Quicksilver and Scarlet Witch (see "Quicksilver and Scarlet Witch in 'Avengers 2'").
 
Speaking with IGN, Whedon talked about what he liked about Quicksilver and Scarlet Witch: "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."
 
Whedon also indicated that Iron Man, in spite of whatever happened in Iron Man 3, will answer the Avengers call in the sequel, saying "The question is if the Avengers are called, does he show up?  And the answer is "Yes!"
 
Now whether Iron Man will be played by Robert Downey, Jr. in Avengers 2 could be a different matter with negotiations reportedly ongoing between the charismatic star and the notoriously tight-fisted (by Hollywood standards) Marvel Studios.  The enormous success of Iron Man 3, which has now surpassed $1 billion worldwide, certainly should strengthen Downey’s hand in the negotiations.