That is plausible, if you so wish.  I have a very small bio of the Southern King in the Characters tab now, if you want to check it out.

But mind you, this cousin might have remained hidden since the Aurrians would have killed off anyone who could have proposed legitimacy to the throne.  They didn't want anyone claiming bloodlines, else they'd risk an insurrection.  That's one reason the Exiled King fled to the southern continent, so he could carry his bloodline on.