I think it will make a difference because BRICS nations cannot sanction one another, and that means Cuba will gain access to a huge market that the US has no control over.
They already have "open trade" with BRICS (and most other) countries. The problem is most companies in most countries will not trade with Cuba because they don't want to be targeted by sanctions that will make it impossible to trade with literally most of the companies on the planet. So in other words, this doesn't make a difference.
They already don't have sanctions on Cuba. Companies aren't going to start not complying with US sanctions now that they're in BRICS.
And the issue for Cuba is that other countries are so far away that shipping becomes extremely expensive, because the cargo ship companies won't ship to Cuba.
Just look at all the trade BRICS countries are doing with Russia right now. Also, if the trade is done outside western financial system, then the US can't even tell what companies are trading with Cuba in the first place. Meanwhile, China is already doing tons of trade with Latin America, so it's not like ships have to go just to trade with Cuba.