I know that can be a signal of respect to an older person, my own country does the same thing. Although once you are a grown up you tend stop using it unless is someone really, really close. So I my just be seeing things from a personal frame
Sure there are many answer, we are after all trying to guess which direction Ocean will go, and as always information is scarce. Uncle may refer to a actual uncle or some male figure to young Nika, I am just saying that if X - William is Nika uncle- is true so Y - Nojiki lied about his parent - must also be true so that the story can make sense. And from that try to figure out with direction Ocean may go, based on the questions it raises.
we may also took other lines of thought. If William is not Nika's uncle but a family friend is true, Nojiko may not have lied about either uncle or parent. but the questions it raises are very different from the ones that do if he truly is, and so are the direction Ocean may go.The uncle may not be William at all, though less likely, and so it raises very little questions and have no impact on the story. My point be is that if we accept something like William is the uncle we need to think how it can affect the story, regardless of what Nika meant from uncle.