What does Mary (his sister) do? Why?

Mary is Junior’s older sister whom he nicknames "Mary Runs Away". A promising student and aspiring writer, Mary retreated into her parents’ basement after graduating from high school. However, once she sees Junior pursuing his dreams by going to Reardan, she marries a professional gambler shortly after meeting him and moves to a reservation in Montana. Mary professes to be happy with her new life, which is sadly cut short after a fire consumes the trailer in which she and her husband are living.