What is Brother-in-law?

Legal Definition
Domestic relat. The brother of a wife, or the husband of a sister. There is no relationship, in the former case, between the husband and the brother-in-law, nor in the latter, between the brother and the husband of the sister; there is only affinity between them. See Vaughan's Rep. 302, 329.
-- Bouviers Law Dictionary
Legal Definition
A wife's brother or a sister's husband. There is not any relationship, but only affinity, between brothers-in-law. Farmers' I & T. Co. v. Iowa Water Co.. (C. C.) 80 Fed. 469. See State v. Foster, 112 La. 533, 36 South. 554.
-- Black's Law Dictionary