The unwritten part of the novel with the stepson: writers won't write about that

Mature brunette Dana is a writer who lives with her businessman husband and his stepson Ricky. The boy has recently shown sexual interest in her, and the woman realizes that she wants him too. Her husband is constantly away from home, and to prevent incest from happening, Dana writes down her experiences and fantasies in a new novel. Soon Ricky finds it, reads it, and... suggests that his stepmother do something that can only be written about in an 18+ novel.