The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

brother-in-law
    n 1: a brother by marriage

Bouvier's Law Dictionary, Revised 6th Ed (1856):

BROTHER-IN-LAW, domestic relat. The brother of a wife, or the husband of a
sister. There is no relationship, in the former case, between the husband
and the brother-in-law, nor in the latter, between the brother and the
husband of the sister; there is only affinity between them. See Vaughan's
Rep. 302, 329.