Possible options:

"The Sniper" by Liam O'Flattery. The story is about a sniper who comes under an enemy sniper's fire.  The protagonist eventually kills the other sniper.  As the protagonist heads back to his troops, he stops to look at the enemy sniper only to find out it's his brother.  This is situational irony because what is expected is that the enemy sniper is actually an enemy, not someone the protagonist is supposed to love.