What type of brother does O'Flaherty mean in "The Sniper" in the last line when he writes, "and looked into his brother's face"?

I think that you can say that they are brothers in a
couple of different ways.  I do not think that there is any evidence in the text to say
that one or the other of these is wrong or right, though.

I
have always taken this literally.  I believe that the enemy sniper was the actual
brother of the sniper from whose point of view this story is told.  I am just taking
this from the literal words of the last line.

However, you
can also argue that the word brother is metaphorical.  After all, these two guys are
doing the exact same job, just on different sides of the conflict.  It is said that
soldiers on opposite sides of wars have more in common with each other than with their
own people who have not been in combat.  You can say that this is what is going on with
these two snipers -- they are brothers because they are both in the same
role.

I tend to believe that they really are brothers.  I
say this because I think a major point of this story is that these conflicts are
needlessly turning people against each other when they really are similar people. 
Having the two be brothers is a way of dramatizing this dynamic.