A niece's son calls his uncle his uncle, just like everyone else.
The niece's son calls her uncle his great uncle. He is the great uncle's great nephew.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
You would refer to them as your nieces, thought they are formally your wife's nieces. And they would normally call you uncle.
Your brother's children call you their aunt if you are female, or their uncle if you are male. Their are your nieces and nephews.
Your son will be a nephew to your brother, and will call him "Uncle".
Your Great Uncle's son would be your 2nd cousin.
The children of your husband's sister are your nieces or nephews, and you are their aunt, just as he is their uncle.
Boys = "nephews", Girls = "nieces". If you are a man they will cal you "uncle" If you are a woman they will call you "aunt" or "auntie".
He would be your grand nephew; you would be his great-aunt.
Your cousin.
Children of your siblings are nieces and nephews. Children of your spouse's siblings are still called nieces and nephews, but you are not related to them by blood.
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).
Your son is your uncle's grandnephew. Your uncle is your son's great uncle.