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.
Brother-in-law
Your younger brother's wife's sister's daughter would be your son's cousin.
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.
First cousins once removed.
Your sister-in-law's husband may be your brother. If so then the brother-in-law is either your husband, in which case the daughter is your daughter; or the brother-in-law is your brother, so the daughter is your niece or the brother-in-law is the brother of another sibling's spouse, in which case the daughter is still your niece If your sister-in-law's husband is not your brother, then he is not related to you and his brother-in-law's daughter is not related to you.
Your brother or sister's daughter is called 'your niece.'
Your sister's daughter would be considered your niece.
Your wife's brother is your brother-in-law.
The relationship to you would be your cousin !
Sister in law
you are referring to a sisters father, his daughter
They were brother and sisters.=Mary was born first. Then Elizabeth and Edward last.=