In the short story 'The Sniper' by Liam O' Flaherty there are least two interpretations for the term 'brother.' As well as being brothers by blood, many of the Irish freedom fighters in Ireland at the time of the Easter Rising owed their heritage to the 'Fenian Brotherhood.' Strangely enough, many of the United Irishmen of the century before were not even Irish,let alone Catholic. They were fair-minded professional men,some anglo-irish and some from other countries who wanted a fair outcome for their Irish 'brothers.' Sadly, many of the early'risings' or mini-rebellions were easily put down, and the voices of the brotherhood didn't get get listened to. The bloodshed and executions caused many of the fighters to become 'blood-brothers' if not brothers-in-arms. Some links with info on the Fenian Brotherhood below:
The word "brother" can be interpreted in different ways, but I don't think it really changes "The Sniper's" theme much. When we are looking for a theme, we are looking for the author's message that is understood by reading the story. Sometimes that is easy and sometimes it is hard (and sometimes there really isn't much of a theme.) "The Sniper," though, falls under the category of "easy" (largely because of that last line.) The story, at heart, is about the horrors of war and how a civil war, in particular, can divide and destroy families. There are others, but I think those are the main ones.
Now, what are the different ways that we can interpret the term "brother?" Well, the most obvious (and the one I think O'Flaherty intended) is to say that one person is related to another, as in legal family member. The other, looser interpretation would be to say that the term brother referred to an "unofficial" brother, the kind you get through a mental, emotional, or physical closeness. The kinds of situations that breed this type of "brotherhood" are close friendships, serving in the same military unit, going through some intense experience together...you get the idea. In this case, if we use the "loose" definition of brother we would be saying that the Sniper feels like the man he shot was his brother in spirit if not by blood.
Are you still with me?
My point is that either way that you look at the term "brother" the main point of the story (about the horrors of war, and its dividing effects) doesn't change. The Sniper is horrified because he has shot and killed someone very close to him.