Your mother's brother is your uncle.
Any child of your mother's brother is your first cousin.
Your brother's daughter has a daughter? That is a great niece.
Your daughter is your brother's niece. Your brother is your daughter's uncle
Your brother's daughter is your niece. You are her Aunt or Uncle.
Your mother's son is your brother or your half brother.
Cousins
The relationship to you would be your cousin !
it will be your niece......
Your brother's daughter is your niece..and you are her AUNT.
Your brother's daughter and your daughter are first cousins to each other. Your brother's daughter and your daughter's daughter (your granddaughter) are first cousins, once removed, to each other.
Your sister's daughter and your sister's brother are niece and uncle to each other. The daughter is the niece of the brother and the brother is uncle to the daughter.
If the brother-in-law's daughter is also your sister's daughter, she would be a niece. Otherwise, there is no relationship.
neice