its called a niece
Your brother or sister's daughter is your niece.
You brother's daughter is your niece, just as your sister's daughter is your niece. The English language does not have different terms for daughter of brother and sister.
brother in law
Your niece.Your brother's daughter would be your niece and you'd be her uncle or aunt. Your sister's daughter would also be your niece. If your sibling had a son, he would be your nephew.
You call him your nephew. (pronouced nes-few)
The daughter of your brother or sister is your niece.
Your brother's sister is your daughter's aunt, and your daughter is your brother's, and his sister's, niece.
Your brother or sister's daughter is called 'your niece.'
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.
I would think a niece, it's no different then a sister's daughter. But I'm not sure
If your uncle is the brother of one of your parents, his sister-in-law is either the sister of his wife or the wife of his brother. If his sister-in-law is the sister of his wife, she is not related to you, and neither is her daughter. If his sister-in-law is the wife of his brother, then she could be your mother and her daughter would be your sister, or you if you are a girl. Otherwise she is your aunt (the wife of another brother of one of your parents) and her daughter is also the daughter of a brother of one of your parents and thus your first cousin. If the daughter is from a previous marriage, then she is not related to you, but you may choose to treat her as a first cousin, especially if she was very young when her mother married your uncle.
If your brother-in-law is your spouse's brother, then his sister's daughter is your spouse's niece, and your niece, too. If your brother-in-law is your sister's husband, then his sister is not related to you, and neither is her daughter.