Source : Webster's Revised Unabridged Dictionary (1913)

Sister-in-law \Sis"ter-in-law`\, n.; pl. {Sisters-in-law}.
   The sister of one's husband or wife; also, the wife of one's
   brother; sometimes, the wife of one's husband's or wife's
   brother.

Source : WordNet®

sister-in-law
     n : the sister of your spouse
     [also: {sisters-in-law} (pl)]