Learn English – Relation name for the wife’s sister’s son’s wife

He is your brother-in-law.

There has already been a similar question posted here to which the answer was "sister-in-law", so I'll finish the list here:

Your spouse's brother or your sister's husband is your brother-in-law.

Your spouse's sister or your brother's wife is your sister-in-law.

Your spouse's father is your father-in-law.

Your spouse's mother is your mother-in-law.

Your daughter's husband is your son-in-law.

Your son's wife is your daughter-in-law.

Where applicable (and where disowning has not occurred), you child's same-sex life partner under a legal bond (whether marriage or civil union or what have you) or a bond you recognise even when the law does not would be either a son-in-law or daughter-in-law, as applicable.

According to this book, they are considered "co-wives."