Sometimes people who are not related are called brothers to symbolise solidarity and equality. Especially under extreme conditions when that kind of thing is put to test. Of course you neglect any women (who may be involved) if you use this figure of speech. This is probably why it has come out of fashion. But you get it in old texts. The French revolution famously had the slogan "unité, egalité, fraternité", in German: "Einigkeit, Gleichheit, Brüderlichkeit". This was an honest slogan, because at the time the term "rights of man" was still to be understood literally ;) So about the word "brudermensch": it could be translated literally as brother human; "We are members of the same species, so we should treat each other with respect and care about each other's welfare." - the intended message will probably be something along these lines. The use of this word alone is quite a statement on its own. You also get the word "Menschenbruder", which means the same.