I don't think bre has anything to do with brate..It's just a thing that got stuck in our language without a clear explanation
Momo Kapor - Bre (<--- a must read)
Čudna je ta reč bre
This sounds quite possible to me:
Bre originates from Turkish: be, bre;
be
1. Oh, you! Hey, you! (used vocatively to express reproach).
2. vulg. Hey! (used in a terminal position): Neredesin be? Hey, where are you?
and it came to Turkish from Greek word more, moros, meaning foolish, dull (SR: luda)
Serbian: ajde bre!
In Greek: "ela, re". "Ela" = 'ajde", "re" = "bre"