How should the sniper have reacted to his brother's death in "The Sniper?"emotions of the sniper after his brothers death

This depends on which of the sniper's personas you think
is the real him.  The sniper shows two personalities over the course of the story.  He
shows the one whose eyes have "the cold gleam of the fanatic," but he also shows the one
who is rattled and remorseful after the battle.  These two personalities would have
reacted very differently to the death of his brother.

You
would hope that the remorseful personality would win out.  If the sniper has any real
humanity left, it would surely be activated by the death of his brother.  He should feel
shocked and horrified by what has happened.  However, this horrible event might drive
him deeper into fanaticism.  He might become even more hardened as a way to protect
himself from the emotional impact of having killed his brother.  It is really impossible
to know for sure.  You should decide which of these sounds more plausible to
you.