It depends if your niece's or nephew's parent is your brother or sister.
They are first cousins once removed.
Your daughter-in-law's niece and nephew would be considered your grandchildren's cousins. So, you would be their grandparent-in-law.
He is your uncle and you are his niece/nephew.
The son of your great niece is your great great nephew.
Your niece's son is your great nephew. You are his great aunt or great uncle.
Niece is to nephew as sister is to brother.
Niece would be an opposite as a niece is female and a nephew is male.
Your nephew's sister is your niece.
Your nephew's child is your great niece (if a girl) or your great nephew (if a boy).
Your brother's or sister's son is your nephew. Your nephew's or niece's child is your grand-nephew or grand-niece.
The male counterpart of a niece is a nephew.
If you have a niece or nephew, it is their daughter.