Your son and your nephew are first cousins to each other.
Your son and your nephew's son are first cousins, once removed, to each other.
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 niece's son is your great nephew. You are his great aunt or great uncle.
Your husband is great uncle to your niece's son (your great nephew).
Your niece's son is your great nephew. You are his great aunt or great uncle.
The son of your great niece is your great great nephew. You are his great great uncle.
You son and your niece are first cousins. Your son and your niece's son are first cousins, once removed.
You are their great-uncle/aunt.
uncle and nephew
Your son and your niece are first cousins to each other. Your son and your niece's son are first cousins, once removed.
The son of your great niece is your great great nephew. You are his great great uncle.
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).
the niece because the others are all boys but the niece is a girl