Family tradition is sometimes the worst thing that could ever happen. Kate was raised thinking that she was the chosen one but hated by those in her own family. Her mother had basically taken her life the day she was the one who was picked for this tradition that no one else could be a part of. It had to be on her eighteen year birthday.
When she was forced to do the task that would kill her if she failed. She would be the one who would be able to do it. When she did she was rewarded. With a husband that would be able to give her everything her mother did not want her to have. The reason she was chosen was to help her sisters. Her husband now was the only one who would give it to her and make them all pay.