Joss Whedon previously hinted at a brother/sister team joining the team in Avengers 2. Entertainment Weekly confirms in their latest issue (with an Iron Man 3 cover story) what many thought that Quicksilver & Scarlet Witch will be in Avengers 2.
Apparently since they are both X-Men & Avengers, both Fox (who owns the X-Men in movies) and Disney/Marvel Studios can use the characters. However, it is unclear if Marvel were to use the characters if they could still be mutants or not.
The article also says that Robert Downey Jr. will be back for Avengers 2 as well.