Question

This is one word substitution ( killing of one's brother or sister)

Answer 1

Killing of One's sister - Sorocide

Killing of one's brother - Fratricide

Answer 2

Actually, there are two answers for the above given question.
One is " Sorocide " and the other one is " Fratricide ".
Sorocide : killing of one's sister
Fratricide : killing of one's brother

Hope my answer helps you..
: )