This theory has most likely been proven false one way or another but meh, I feel like nina might be the kings half-sister. They both have red eyes, and nina was abandoned as a child. and it was shown i believe that the kings mother ran away. So what if, when she ran away, she fell for someone else in nina's homeland, had a child, but left her because she didn't want the child to be in danger. I feel like that might be a really wholesome way to wrap up her relationship to the king, the stuff wouldn't be nearly as creepy if she was his little sister. And there is no blood relation to the duke so that romance subplot would still be able to happen, and it would explain why the king and nina have nearly identical eyes. But her hair is blonde.