Emily VanCamp has been cast as Sharon Carter in Captain America: The Winter Soldier.
A report on Deadline claims the Revenge and Everwood actress joins Chris Evans in the Marvel sequel. The following character bio from Wikipedia may give a clue to the plot of the film.
Sharon Carter is a secret agent. She is an ex-field agent of S.H.I.E.L.D under Nick Fury and occasionally the girlfriend of Captain America. In original continuity, Sharon was said to be the younger sister of Peggy Carter, the wartime love interest of Captain America, but she was later retconned as Peggy’s niece. Sharon served a brief term as Executive Director of S.H.I.E.L.D., during the absence of Nick Fury. She then became a field agent once more, reporting directly to new Executive Director Maria Hill as a liaison officer specifically assigned to support and report on Captain America’s activities. While investigating the whereabouts of Jack Monroe, she was abducted by the Winter Soldier, and used as bait to lure Captain America to a trap set by General Aleksander Lukin. She and Captain America later resumed their relationship while on a field mission investigating the activities of the Winter Soldier.
Captain America: The Winter Soldier is due for release on April 4 2014.