Quicksilver and Scarlet Witch confirmed for Avengers 2

Joss Whedon (The Avengers) has confirmed that the brother-sister team that will be included in The Avengers 2 will indeed be Quicksilver and Scarlet Witch.

There was a potential legal issue with these characters as they are intertwined with X-Men seeing as they are both mutants and their father is Magneto. The concern was since Fox owns X-Men they could potentially own these characters as well. However, Whedon assured everyone that the characters are available for them to use and they intend to do so.

If you know nothing about Quicksilver or Scarlet Witch, Quicksilver has a power of super speed. Scarlet Witch can cast hexes which can alter probability, alter reality, and chaos magic. The siblings also have several years worth of story arcs so it isn't possible to really predict exactly which arc Whedon is targeting for The Avengers 2. There is also a rumor that Saoirse Ronan is in the running (aka, the prototype) for Scarlet Witch, but that is pretty preliminary.