Your cousins are also the cousins of your siblings. So, your cousin and the parent of your niece (either your brother or your sister) are first cousins. Therefore your niece is the first cousin once removed of your cousin. (Your cousin's child and your niece are second cousins.)
Your grandson and your niece are first cousins once removed.
No, because the neice is the daughter of the uncle's sibling. If the uncles' sibling didn't have a child, then the uncle wouldn't be called "uncle" since there is no neice.
Your niece would be the daughter of your sister or brother. Your nephew would be the son of your brother or sister. Their children would be your great nieces and great nephews.
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.
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.
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 your uncle's niece. You and your uncle's children are first cousins to each other.
great niece
Your sister's daughter is your niece. If you are a woman, you are your niece's aunt; if you are a man, you are your niece's uncle. Your sister's son is your nephew.