A sarcastic, disingenuous expression indicating appreciation for another person's boast. Although "sick" usually modifies an expression with awesomeness, the "bro" effect is stronger, dominating and suppressing any notion of awesomeness.
Note: Not to be confused with "sick bag, bro", which mocks someone who brags about sleeping with an unattractive person.
John: Then he check-raised me allin. I realized immediately from the line he took that he was bluffing (likely a busted flush draw), so I called. He turned over the busted draw, and I won a huge pot! Isn't that awesome!?
Wally: Sick brag, bro.