Your brother or sister's daughter is called 'your niece.'
If it is your brother's child, she would be your "niece." If the daughter is not fathered by your brother, then she is your "sister in law's daughter"
If you are married to your daughter's mother, then her boy is your stepson. If you are not married, then he is no relation to you. Either way, he is your daughter's half brother.
If the brother-in-law's daughter is also your sister's daughter, she would be a niece. Otherwise, there is no relationship.
its called a niece
Your mother's brother is your uncle. Your uncle's daughter is your first cousin. Any child of your first cousin is your first cousin, once removed.
shes called ur cousins daughter
A niece. How could you not know that
The English language does not include a relationship called "co-sister-in-law" or one called "co-sister." If you are living in another culture and are trying to describe a relationship different from that of basis "sister-in-law," you will need to be more specific. If your sister-in-law is the wife of your brother, and her daughter is also your brother's daughter, then you are the daughter's aunt or uncle and she is your niece. If she is not your brother's daughter, then you are not related to her. But you can treat her as your niece if you want to. If your sister-in-law is the sister of your spouse, then her daughter is your spouse's niece and yours as well since in English we talk of "aunt and uncle" not "aunt-in-law and uncle" or "aunt and uncle-in-law."
Your husband's niece is your niece, too. You and your husband are her aunt and uncle. Your brother is not related to a niece of yours who is the daughter of your husband's sibling.