'Sister-in-law' definitions:

Definition of 'sister-in-law'

(from WordNet)
noun
The sister of your spouse

Definition of 'Sister-in-law'

From: GCIDE
  • 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]