Your son and your niece are first cousins.
Your husband is great uncle to your niece's son (your great nephew).
Your niece's son's daughter is your great grand niece. Your niece's son is your grand nephew or your great nephew, whichever term you prefer.
great nephew
Your grandfather's niece is a first cousin of one of your parents. Her son is your second cousin.
uncle and nephew
cousin in law.
Probably your niece-in-law.
That's easy cousins.
Your brother's son or daughter is your nephew or niece respectively.
There probably isn't a relationship. You do not share a common ancestor.
Your half-brother's daughter is your niece. Your niece and your son are first cousins to each other.
Your niece's son is your grand-nephew. You are the child's grand aunt, or grand uncle (whichever applies).