Deadline reports that the exact nature of VanCamp's role is being kept under wraps, although in the comics she plays the niece of Peggy Carter, Captain America's love interest from The First Avenger and star of Marvel's Agent Carter. Sharon was Captain America's love interest for much of the last twenty years, including the Ed Brubaker run, from which The Winter Soldier is adapted.
Following the events of Civil War in the comics, Captain America was dead -- murdered by a brainwashed Sharon Carter, who was also pregnant with his child.
...Yeah, comics are a lot like soap operas.
Captain America: Civil War will be directed by Joe and Anthony Russo, who will also helm the next two Avengers movies following Age of Ultron. The movie is in theaters on May 6, 2016.