It was never implied in the novel that he molested her! On the last chapter Regan/demon states that Burke went into her room because he heard noises and moans coming from it! He went up and Regan took him by the throat, screaming that he was diddling her mother and that he caused the divorce.
Why do people want to believe that Burke was sexually abusing her??
Because of the victim mentality and paranoia present today..people see a child molester where one doesn’t exist.