Princess Rosette was not stupid, she was quite intelligent in fact, but sometimes she was just clueless. There were a number of completely obvious things that she missed altogether, mostly involving her daily lessons. She noticed everything about people though.
She noticed how her best friend, Lily, stared at her brother just a little longer than necessary sometimes. She noticed the frightened look behind her father's kind expression, and the cruelty behind her younger sister's adoration, the unease in her mother's mannerisms when the neighboring country, or her uncle's "habits" were mentioned. She noticed her brother, Kieran. She noticed how stubborn he was, how upset he seemed when she mentioned the things she saw, how little he hid from her. He told her why the things she noticed were there, even when he didn't want her to know. She noticed how his obstinacy on a matter disappeared when he saw she supported it. She noticed how everyone admired her brother, and how people seemed to look more kindly upon her in his presence.
Princess Rosette was very intelligent for noticing that as long as she kept the favor of Prince Kieran she was safe.