BRICS currency is something meant for international trades on very large scales (trillions of dollars) between countries that is currently taking place using dollar. That is why its creation will have a negative impact on US dollar as it will lose its demand and with dollar's massive circulating supply and US unimaginable national debt the dollar value could crumble.
My guess is that there won't happen much. The US stands for stability, or at least for more stability than the BRICS States.
BRICS States are a club of paria States. Their wheeling and dealing is not that big. Take away China and BRICS is nowhere near their numbers.
Why do you think it is trillions of $ in trades?
South Africa is having not even 1% of the trade:
https://oec.world/en/profile/bilateral-country/usa/partner/zaf