What does James Stockdale mean in You Can Find the Courage when he says he was his brother's keeper?

What he is saying here is that he was not selfish -- he did not think only of himself.

When we say we are not our brother's keeper, we are saying we do not want to be responsible for helping others.  We are saying we only want to think of ourself.

But Stockdale is saying he did the opposite.  He does not say exactly what he did.  He says that the prisoners' unwritten law said that the most important thing was to support the other prisoners even if you might be able to get something better for yourself by being selfish.