Source : Webster's Revised Unabridged Dictionary (1913)

Brother-in-law \Broth"er-in-law`\, n.; pl. {Brothers-in-law}.
   The brother of one's husband or wife; also, the husband of
   one's sister; sometimes, the husband of one's wife's sister.

Source : WordNet®

brother-in-law
     n : a brother by marriage
     [also: {brothers-in-law} (pl)]