The USD didn't really become the world reserve currency until after WWII when the European empires fell apart and the US was a clear superpower.
It'll be interesting to see what happens now. I suspect the Russians will play silly buggers because they seem to enjoy that. And they have significant oil to play silly buggers with.
I have no doubts they'll all increase trade together, including military hardware, but establishing a reserve currency requires trust and dependency.
Those seem heavy loads for the ties of pragmatism and convenience that bind BRICS together.