Insiders claim that the actress is playing the daughter of Hank Pym (Michael Douglas), who is also the love interest of Scott Lang (Paul Rudd). Hank Pym didn't have a daughter in the Marvel comic books, so it's possible that she may either play someone specifically created for the movie, or an existing character whose history has been altered. It should also be noted that, in the comics, Scott Lang dated private investigator Jessica Jones, before he learned that she was carrying Luke Cage's child. We reported in November that Marvel is developing TV shows centered on Jessica Jones and Luke Cage for Netflix.
We reported in October that Rashida Jones is the front runner to play Janet Van Dyne, a.k.a. The Wasp, who an insider said is "a love interest and a major supporting character" in Ant-Man. Of course, it's possible that Evangeline Lilly is playing a different character, and that Marvel is still deciding on who will play Janet Van Dyne, but that has not been confirmed.