@AABSorry! I didn't mean for there to be such a mix up. The Aunt princess would most
definitely be born before the King ascended. Also, since the rp is planning to start the morning before the king is murdered, I plan to make a smaller CS for him. You can name the deceased Queen if you like.
If we go with the plan you have for your character's father to be alive, only if he went to another kingdom would that mean that she would get to stay at the castle. There could be even the smallest chance for the girl to have magic power and he couldn't let her go out of the country unless he was certain she didn't have any power, which he hadn't met her so he didn't know.
Besides that, this is also medieval times, so teen pregnancy is really nothing at all, but whateves.
EDIT: Also, the niece is brought specifically because the King believes he has no alternative.