http://www.cinemablend.com/new/Samu...-Scarlet-Witch-Avengers-Age-Ultron-39668.html
Samuel L. Jackson confirms that Olsen will play Scarlet Witch in this interview.

Samuel L. Jackson confirms that Olsen will play Scarlet Witch in this interview.
Since word first arose that Joss Whedon would be bringing in the brother-sister act of Scarlet Witch and Quicksilver to Avengers 2, all corners of the internet have been speculating on who might land these coveted roles. Early on Saoirse Ronan was offered the part, but turned it down. By end of summer indie It girl Elizabeth Olsen was rumored for Scarlet Witch, and her casting has now been confirmed by the one and only Samuel L. Jackson. While discussing his cameo on Marvel’s Agents of S.H.I.E.L.D with The Wall Street Journal, Jackson spoke about the upcoming Avengers sequel The Avengers: Age of Ultron and (name)dropped this casting confirmation:
”I know we’re shooting in London, that James Spader is Ultron and going to be the bad guy, and that we added Ms. [Elizabeth] Olsen [who will play the Scarlet Witch], but I don’t know what she’s doing, if she’s on the inside or the outside. I haven’t seen a script.”