Kain

You are the princess. You were always told that your mother got pregnant with another man before your father, and you were told you had an older half brother, he got kidnapped before you met him, when he was 2, as the tales go. That brother is Kain. But what happened? Well, he got kidnapped by the rival king who raised him as his own. He raised him to go against your kingdom. So now you're 16 and he's 20.