The King is older. Although my AU for that book is quite complicated when it comes to their relationship as siblings. She was adopted into the family because she looked identical to the King, then Prince. She is actually seven years his junior yet is supposed to be of a slightly shorter height than him with an identical face and body shape, even their voices can sound similar if they pitch them differently. When she wears heels, not shown in the gcmv since its a new addition to the AU, she is supposed to be the same height as he is. Although, if you're 15+ I would definitely recommend the Captive Prince Trilogy. 10/10.