(1) Your niece is the daughter of your sibling
(2) Your niece and your children are 1st cousins
(3) Your grandchildren and your niece's children are 2nd cousins
(4) Your g'children and your niece's g'children are 2nd cousins once removed
The granddaughter of your niece is your great great niece.
Your grandfather's niece is your parent's first cousin. Her daughter is your second cousins and her granddaughter is your second cousin, once removed.
Your niece's granddaughter is your great great niece. You are her great great aunt (if you are a woman) or her great great uncle (if you are a man).
Your granddaughter and your niece's son (the grandson of one of your siblings) are second cousins to each other.
Your granddaughter and your niece's son are second cousins.
Your brother's daughter is your niece and his granddaughter is your great niece.
Your grand-niece.
Your niece's granddaughter is your great great niece. You are her great great aunt (if you are a woman) or her great great uncle (if you are a man).
Your granddaughter is your sister's great niece. Your sister is your granddaughter's great aunt.
Your niece would be your mothers granddaughter.
Your brothers great granddaughter is your great great niece.
Your granddaughter could be your son's daughter, or your son's niece.