The granddaughter is the niece's first cousin once removed.
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 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 grand-niece.
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.
Your 1st cousin's granddaughter would be your daughter's niece.
They are first cousins once removed. Your niece's children and your grandchildren will then be 2nd cousins.
Your aunt is great great aunt to your son's daughter (your granddaughter). Your granddaughter is your aunt's great great niece.
Your sister's daughter is your niece and her granddaughter is your great niece.