So far there is no codified admission process, so nobody has official veto powers. But in practice, the very first expansion is based on concensus of the existing five, that is nobody objects anyone, so it is kind of veto power of the initial five members. But again, as the orgnization expand, there will be more members needed to reach concesus, some applying members are even more economically influensial than existing members. I don't think there will be real veto powers to anyone including some of the funding members, let alone others. It will be up to a majority to reach a concensus.
So in short, I don't think any possible new members including Turkey to have veto right. What would happen is for example, economy the size like China may have a de facto veto due to her size and friend circle in BRICS, while economy the size of Tureky is not able to gather enough support to veto anyone who has the endorsement of China. It is all based on hard power, the word of "consensus" is smart choice to balance the interests of both large and small members. Think of BRICS as a company, the big shareholders will naturally have a bigger say and they won't let others be equally deciding their investments.
