Who is My Brother-in-Law?

A "brother-in-law" could easily be the husband of your sister, your wife's brother, or your wife's sister's husband. In certain times, a "brother-in-law" could also be what today we would consider a "step-brother."

Don't assume the specifics of the relationship until you've got evidence indicating the specifics of the relationship. At its most generic, a "brother-in-law" relationship is a "brother" relationship created by someone's marriage.