A nephew is a cousin to his aunt's son.
a nephew's kid to his aunt's kids would be their cousin.
Great Grand Nephew
You are their aunt or uncle and they are your niece or nephew.
You are her niece or nephew (niece if female, nephew if male) and she is your aunt.
Your nephew's daughter is your great niece and you are her great Aunt (or Great Uncle).Correction:Your nephew's daughter is your grand niece. You are her grand aunt or grand uncle.
His Aunt
He is the nephew and you are the uncle or aunt, depending upon your gender.
Your grandson is your sister's great nephew and she is his great aunt.
Your sister's son is your nephew. You are his aunt, if you are female, or his uncle, if you are male.
Your sister's son is your nephew. You are his aunt if you are a woman, or his uncle if you are a man.
That's easy cousins.