In English, the word ‘brother’ in military context, is a ‘term of art’, meaning that one will remain loyal under duress. (See Shakespeare). In other words, you can trust one another to ‘cover your ass’ on the battlefield. We often claim that this extension of kinship ‘love’ to non-kin has it’s origins in Christianity. But it was the first principle of the initiatic brotherhood of warriors from which all monotheistic religions evolved. So no, the extension of kinship love was dominant in militial prehistoric, and pre-christian europe, and the christianization of it as a means of rebellion against aristocracy by resistance using submission, was merely an attempt to replace the heroism and loyalty of Achilles and his tragedy, with the submission of jesus and his tragedy.
The universe is a very simple place. Humans are simple creatures. Our problem is not so much in discovering new truths, but in discarding old lies.