they all say "brothers and sisters" must be code for something...a verbal secret handshake
My 2 cents -- It goes at least back to the International Workers of the World -- IWW -- Wobblies. As you noted, a verbal handshake. The organizing songs from the teen's through the 40's were heavily into "Brothers and Sisters". Pete Seeger still uses it. I suppose it can be said to go back to the tent revivals. That is the "Unionists" trying to borrow the thunder of the evangelists. In the early days of the Unions, the same audiences tended to go to both types of meetings.