Brother-In-Law A wife’s brother or a sister’s husband. There is not any re-lationskip, but only affinity, between broth-ers-ln-law. Farmers’ L. & T. Co. v. Iowa water Co. (C. C.) 80 Fed. 469. See State v. Foster, 112 La. 533, 36 South. 554 Source: Black's Law Dictionary 2nd Ed (1910)