your sons are family, true, and families have may have a variety of terms of endearment.
Calling Ennis and Jack "the boys" or "family" fictionalizes them, I think, tends to reduce their individualism, separates them from reality, and categorizes them a eternal kids. The major part of the story/film/relationship was a grown, mature men, and in fact, the essence of the work is about maturing, growth, and manhood.