Responses
Nia calling hilna's father uncle might just have been him implying they're close since in some countries/languages calling someone by a word like uncle, aunt, sister or brother is a form of expressing familiarity, not saying that they are family.
I'm pretty sure there were many instances where it was made obvious that Sienna should kinda butter up to Nya and marry him, BUT he has also called the emperor of the western empire "uncle". That would mean that Nya and Sienna would be cousins... I know that this was common in such times because the families did not want other blood mixing with their pure family's one but all I can say is SWEET HOME ALABAMA