cousin-in-law
Your daughter's uncle can be your brother. Her parent who is not your sibling (your brother-in-law or sister-in-law) could also have brothers. They will also be your daughter's uncles, but will not be related to you.
If the brother-in-law's daughter is also your sister's daughter, she would be a niece. Otherwise, there is no relationship.
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.
Use of their name is appropriate. If you have to explain their relationship, they would be your son's wife's brother, or your daughter-in-law's brother.
Daughter in law
She is the co-mother in law of you and your daughter's father in law is your co father in law. :-)
your grand niece or nephew in-law (-Kasadie)
She is the co-mother in law of you and your daughter's father in law is your co father in law. :-)
grandma in law.
Daughter is the opposite of son.
Son-In-Law
Daughter