depands if on your side or hers
Your niece's daughter is your great niece. You are her great aunt or great uncle, depending on your sex.
My niece's daughter is my great niece.
Nieces' is the plural possessive form of the noun niece. The singular possessive form would be niece's.
Yes, your niece's daughter is your grand niece. You are her great aunt or great uncle.
Your son and your niece are first cousins to each other. Your son and your niece's son are first cousins, once removed.
Your children are the cousins of your niece. Your niece's children would be second cousins to your children, and vice versa.
Your niece is the daughter of your sister. If you have more than one sister, you can have nieces who are also the nieces of your sister, since they are daughters of the other sister.
You would refer to them as your nieces, thought they are formally your wife's nieces. And they would normally call you uncle.
Your niece's daughter is your grand niece, or, as some would say, your great niece.
Your nephew's children are your great nephews and great nieces (or grand nephews and grand nieces).
There is no special term in the English language to describe a relationship between you and your nephew's wife. To her, you are just "my husband's uncle." If she has children with your nephew, they will be your great nephews and great nieces and you will be their great uncle or great aunt.
If you are a woman, you are great great aunt to your niece's granddaughter. If you are a man, you are great great uncle to your niece's granddaughter. In either case, your niece's granddaughter is your great great niece.