Your child and your sister's daughter are first cousins.
Your grand child and your sister's daughter are first cousins, once removed.
Your sister's daughter would be considered your niece.
The relationship to you would be your cousin !
Great Granddaughter
No. This is a religious relationship, not a family relationship.
She is your niece, you are her aunt or uncle.
Your granddaughters and son have the relationship of nieces and uncle. Your granddaughters being his nieces and your son being their uncle.
Some exact nouns for two children are sisters, brothers, brother and sister, cousins, sons, daughters, son and daughter, grandchildren, grandsons, granddaughters, etc.
Your aunt is the sister of either your mother or your father. Your aunt's sister is either:your mother, and her daughter is you (if you are a girl) or one of your sisters, oranother one of your mother's sisters, and her daughter is your first cousin, or,one of your father's sisters and her daughter is your first cousin.
First cousins once removed.
If he is your brother in law then his sister is your sister in law. So your sister in law's daughter would be your niece.
You are an uncle or aunt, depending on your sex. If 'y' meant you - I've no relationship with her at all
Your younger brother's wife's sister's daughter would be your son's cousin.