Mike
My Concise Oxford Dictionaries (Fifth & Ninth Edition) both define it as 'the killing of one's brother or sister'. It throws up an interesting question, doesn't it. Where do you go for the definitive definition of a word? Mike, I guess you have a source that defines it differently.
Regards
John
<Added>There's an article from the Los Angeles times at
http://www.globalsecurity.org/org/news/2003/030516-friendly-fire01.htm which discusses 'friendly fire' and uses the word 'fratricide' throughout!