When Juniors sister move to Montana

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. I think Junior is very skeptical that his sister is happy or achieving any of her goals.