cousin, niece, nephew, aunt, uncle, grandparent
Your sister's granddaughter is your great niece and you are her great aunt (if you are a woman) or great uncle (if you are a man).
Your niece will call you her aunt or uncle.
uncle, niece
Your niece's children are your great nieces and great nephews, and you are their great uncle. Your uncle could be the father of your niece, and thus the grandfather of your niece's children.
A niece's son calls his uncle his uncle, just like everyone else.The niece's son calls her uncle his great uncle. He is the great uncle's great nephew.
You are her great-aunt/uncle. She is your great-niece.
The great niece and the great uncle's son are first cousins once removed. Many cultures, but not all, would consider this to be too close a relationship for marriage.
You are the great-great-aunt/uncle to your niece's granddaughter. This means that your niece's granddaughter is your great-great-niece. The term "great-great" signifies that there are two generations between you and your niece's granddaughter.
You are your uncle's niece. You and your uncle's children are first cousins to each other.
great niece
'Niece' is what a little girl is to her uncle. 'Nephew' is what a little boy is to his uncle. The children also are the niece and the nephew of their aunt.