context: Ren McCormack, a teenager raised in Chicago, moves with his mother to the small town of Bomont to live with his aunt and uncle. Soon after arriving, Ren befriends Willard Hewitt, and from him learns the city council has banned dancing and rock music. He soon begins to fall for a rebellious teenage girl named Ariel, who has an abusive boyfriend, Chuck Cranston, and a strict father, Shaw Moore, who is a reverend of the local church.
  After trading insults with Chuck, Ren is challenged to a game of chicken involving tractors. Despite having never driven one before, he wins. Rev. Moore distrusts Ren, and he grounds Ariel, forbidding her to see him. Ren and his classmates want to do away with the no dancing law and have a senior prom. He drives Ariel, Willard, and Ariel's best friend, Rusty, to a country bar about 100 miles away from Bomont to experience the joy and freedom of dancing, but once there, Willard is unable to dance. With a little help from Ren, he eventually catches on.
  Ren goes before the city council and reads several Bible verses to cite scriptural support for the worth of dancing to rejoice, exercise, or celebrate. Although Reverend Moore is moved, the council votes against him. Vi, Moore's wife, is supportive of the movement and explains to Moore that he cannot be everyone's father and that he is hardly being a father to Ariel. She also says that dancing and music are not the problem. Moore feels betrayed that even his wife does not believe in him even though she assures him that she always did.
  Despite further discussion with Ren about his own family losses in comparison to Moore's losses and Ariel's opening up about her own sinful past, even going so far as to admit that she has had relations, Moore cannot bring himself to change his stance because his son Bobby was killed in a car crash, resulting in banning music and dancing in the community. Soon, however, he has a change of heart after seeing some of the townsfolk burning books that they think are dangerous to the youth. Realizing the situation has gotten out of hand, Moore stops the burning.
  The following Sunday, Moore asks his congregation to pray for the high school students putting on the prom which is set up at a grain mill outside of the town limits. Shaw and Vi are seen outside, dancing for the first time in years.
question: WHO IS BOBBY SISTER?
answer: Ariel