Cruel Intentions

In an upscale New York City mansion, wealthy and popular teenager Kathryn Merteuil (Gellar) is discussing her prep school with Mrs. Caldwell (Christine Baranski) and Mrs. Caldwell's daughter, Cecile (Selma Blair). Kathryn promises Mrs. Caldwell that she will look out for the sheltered and naÃ¯ve Cecile. Kathryn's step-brother Sebastian (Phillippe) enters the room, whereupon Mrs. Caldwell reacts to him coldly and leaves with Cecile. Kathryn reveals to him that her real intention is to use Cecile to take revenge on her ex-lover Court Reynolds (Charlie O'Connell), who has dumped her for Cecile. Kathryn asks Sebastian to seduce Cecile; he refuses as he is planning to seduce Annette Hargrove (Witherspoon), the virgin daughter of their prep school's new headmaster. Annette is a 'paradigm of chastity and virtue' who recently wrote a published essay about saving herself for marriage and has been temporarily staying with Sebastian's aunt. The two make a wager: if Sebastian fails to bed Annette, Kathryn gets Sebastian's vintage Jaguar XK140; if he succeeds, Kathryn will have sex with him. It is mentioned that Sebastian keeps a journal detailing his conquests.
Sebastian's first attempt to seduce Annette fails, as she had already been told of his reputation as a womanizer. He vents to his friend, Blaine Tuttle (Joshua Jackson), who suggests that the informant might be Annette's ex-boyfriend and closeted jock, Greg McConnell (Eric Mabius), revealing to Sebastian that Greg tried to make a pass at him. He uses that to seduce Greg while being secretly filmed by Sebastian. Sebastian confronts Greg with the photographs, but he denies warning Annette. Greg is pressured into investigating who did, and Sebastian also orders him to present him in a good light to Annette. Later, while gushing about Sebastian to Annette, Greg discovers that the culprit is Cecile's mother, Mrs. Caldwell. Wanting revenge on the Caldwells, Sebastian agrees to seduce Cecile.
Meanwhile, Cecile's music teacher, Ronald Clifford (Sean Patrick Thomas), is in love with her. Cecile confesses this to Kathryn. Kathryn tells Mrs. Caldwell about Ronald and Cecile's romance and Mrs. Caldwell orders Cecile to end it. Sebastian, in turn, calls Cecile to his house, ostensibly to give her a letter from Ronald. There he blackmails Cecile in order to perform oral sex on her. The next day, Cecile confides in Kathryn, who advises her to learn from Sebastian so that she can make Ronald happy in bed.
Sebastian begins to fall in love with Annette, who returns his feelings but still resists him. Sebastian calls her a hypocrite because she claims to be waiting for her one true love, but when her one true love chooses to love her back, she resists. She relents, but Sebastian refuses her, confused about his feelings colliding with his stolid sexuality. Annette flees to the estate of her friend's parents. Sebastian tracks her down, professes his love, and makes love to her. As he has won the bet, Kathryn offers herself to Sebastian the next day, but he refuses; he now only wants Annette. Kathryn taunts him and threatens to ruin Annette's reputation, so Sebastian pretends indifference to Annette and coldly breaks up with her.
After Sebastian tells Kathryn that he has broken up with Annette and arranged for Cecile and Ronald to be together, Kathryn reveals that she has known all along that he was truly in love with Annette and manipulated him into giving her up. Sebastian angrily dismisses Kathryn, saying he no longer wants her, and she then rejects him. Sebastian leaves to confront Annette, and Kathryn calls Ronald, telling him that Sebastian slept with Cecile and lying to him that Sebastian hit her. Annette refuses Sebastian's apologies; he sends her his journal, in which he has detailed all of Kathryn's manipulative schemes as well as their bet, and written the true feelings he had for Annette all along. As he heads home, Ronald intercepts him on the street, starting a fight. Annette runs out and tries to stop it, but is accidentally thrown into the way of traffic. Sebastian pushes her to safety, and is hit by a cab. Before he dies, Sebastian and Annette profess their love for each other. Watching this, Ronald realizes that Kathryn lied to him and used him to kill Sebastian.
At Sebastian's funeral, Cecile distributes copies of his journal, made into a book by Annette, titled Cruel Intentions. Kathryn is humiliated and rejected by her former friends, and her reputation is ruined when cocaine is discovered in her rosary. In the final scene, Annette drives away in Sebastian's car with his journal at her side as she remembers the moments they shared.

Question: What realation is Sebastian to Kathryn?
Answer: step-brother