What do you call your brother in law's sons wife?
If your brother-in-law is your wife's brother, his son is your
nephew and you are his uncle or aunt.
If your brother-in-law is sister's husband and his son is also
her son, he is your nephew and you are his uncle or aunt. If the
brother-in-law's son is from a marriage to a woman other than your
sister, and your sister has not adopted him, he is not related to
you. However, you may choose to treat him as your nephew,
especially if he came into your life as a young child and considers
your sister to be his mother.