sister-in-law

from WordNet (r) 3.0 (2006)

sister-in-law
    n 1: the sister of your spouse

from The Collaborative International Dictionary of English v.0.48

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.
   [1913 Webster]