Your mother's niece is your first cousin. Your first cousin's daughter is your first cousin, once removed.
Your niece's daughter is your grand niece, or, as some would say, your great niece.
Grand niece.
cousin
You call them your nieces, or by their names.
my cousons sisters boyfriens dads nieces grandma's favorite sons dogwalkers mothers friends mum twice removed uncles daughter
your great aunt.
Great-niece.
You would refer to them as your nieces, thought they are formally your wife's nieces. And they would normally call you uncle.
Mothers sister is aunt and son is cousin so daughter wouldn't that be a niece
your 2nd niece...!
Your daughter and your niece's child are 1st cousins once removed.
My niece's daughter is my great niece.