The Collaborative International Dictionary of English v.0.48:

Sororicide \So*ror"i*cide\ (?; 277), n. [L. sororocida, and
   sororicidium; soror a sister + caedere to kill.]
   The murder of one's sister; also, one who murders or kills
   one's own sister. --Johnson.
   [1913 Webster]