The Collaborative International Dictionary of English v.0.48:

Fratricide \Frat"ri*cide\, n. [L. fratricidium a brother's
   murder, fr. fratricida a brother's murderer; frater, fratris,
   brother + caedere to kill: cf. F. fratricide.]
   1. The act of one who murders or kills his own brother.
      [1913 Webster]

   2. [L. fratricida: cf. F. fratricide.] One who murders or
      kills his own brother.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fratricide
    n 1: a person who murders their brother or sister
    2: fire that injures or kills an ally [syn: friendly fire,
       fratricide]
    3: the murder of your sibling