Sororicide

Sororicide (from Latin soror "sister" + -cide, from caedere "to cut, to kill") is the act of killing one's own sister.

There are a number of examples of sororicide and fratricide in adolescents, even pre-adolescents, where sibling rivalry and resulting physical aggression can get out of hand and lead to the death of one of them, particularly when a potent weapon is available or one is significantly older than the other and misjudges his/her own strength.