What do you call your sister's daughter?
You would call her your niece
Your sister's daughter is your niece, her granddaughter is your "great niece"... or you could just call her by name.
your sister's daughter= your niece your sister's son= your nephew Your sister's son or daughter would be your nephew or niece respectively.
She is your niece
Your wife's sister is your sister-in-law and her daughter is your niece.
Your sister's daughter is your niece. You are her aunt (if you are female) or her uncle (if you are male).
They are first cousins.
um your cousin
Your sister's husband's sister's daughter might be called your brother-in-law's niece, although that is less specific than calling her your brother-in-law's sister's daughter.… There is no term in the English language for a relationship between you and that person. You are not considered to be related.