Your niece's son is your great nephew.
You are his great aunt (if you are a woman) or his great uncle (if you are a man).
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Your sister's husband is your children's uncle. They are his nieces and nephews.
pride and prejudice
Second Cousin
You would be a first cousin by marriage, in other words, there is no direct relationship between you.
If you are female and have a son, your aunt's niece's son could be you. Your aunt's niece's son could also be your first cousin, once removed, if he is the son of one of your first cousins. Your aunt also may have nieces who are not related to you, and then the son is not related to you either.
There is none. "This is George, my niece's husband." In most families he would referred to as a nephew.
The relationship between them is similar to father and son.
Your niece's son is your grand nephew, and you are his great aunt.
Youre his son.
Your daughters are great aunties if their nieces have kids. Your daughters kids are your grand children.
He is your Grandfather, and is your son's Great-Grandfather.