All that Internet speculation might be right for once after all. EW is confirming that Quicksilver and Scarlet Witch are set to appear in Avengers 2. The characters were first hinted at by Joss Whedon, who mentioned a "brother/sister act" would be appearing in the next film. Most people presumed that Whedon was talking about the Scarlet Witch and Quicksilver, the former of which was responsible for the near annihilation of the mutant race while the latter is a bit of a prick. They're also mutants and the children of Magneto, but don't expect that to come up in the movie, as Fox owns the movie rights to the concept of "Marvel mutants".
While Kevin Feige wouldn't confirm the rumor, he did warn EW that "the draft could change six months from now", referencing the fact that the Wasp was originally going to appear in the first Avengers movie.
So, who do you want to play the pair in the movie? My money's on the Bateman siblings.