huh, so he effectively has her pinned between two causes. If she doesn't do what her brother want, she will be kicked out or possibly killed, or tortured for betrayal. If she does what her brother asks, he will eventually cause her to betray the rebels.
I think the shy sister should be blind, and sickly, which is why her father has her locked up a lot. And this makes the other twin more rebelious. She isn't just a spoiled girl, she has cause to worry. her sister has no freedom because she is blind and sick. and since she is b&s, her marriage opertunities are limited, leading to all the suitors being thrust upon her. With the idea of marriage being just another prison, she rejects the royal life and seeks to assist in the rebellion against her powerful father.