Someone must've hit the mute button on this character. We don't hear Katherine Cross say anything, but she's an extremely important part of the movie: she's the child born of incest by Evelyn and her father, Noah Cross.
Initially, Jake thinks that teenage Katherine is Hollis's mistress, not realizing that Hollis was just looking out for her. Evelyn tells Jake that Katherine is her sister, but he's still suspicious—he thinks that Evelyn is keeping Katherine prisoner because she knows that Hollis was murdered. He doesn't understand that Evelyn is trying to protect Katherine from Noah, and wants to escape with her to lead a new life together.
Of course, it all ends horribly. The police shoot Evelyn after she tries to kill Noah, and Noah takes possession of Katherine. We don't know Katherine's final fate, but since she's fallen into the clutches of a power-hungry madman who conceived her through incest, we can assume it's not so great.