"Bro" is often used between good male friends rather than brothers and some guys use it as a general term to address a male rather than "dude" or "man".
If you are addressing a brother you could say "bro" and for a sister you could say "sis", but I personally wouldn't say "sis" to a girl who was not my sister unless she was exactly like a sister to me (a close lifelong friend that was considered family).
This may be regional, then. Perpend, you no longer list your location, but I think I recall that you're in the Midwest. Perhaps it's more common there.