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.
Then when sisters marry brothers, or a brother and sister marry a sister and a brother ... I am my own brother-in-law!
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