Regarding formation of association

We have purchased a flat from a family of five brothers who have constructed their own building consisting of 11 flats and out of those, 10 flats belong to them and rest 1 flat was sold to us. When we purchased they were joint owners with undivided property. Not sure if they have separated it later or not (I guess they have now separated it by partition deed). Now, they want to register the property under The West Bengal Apartment Ownership Act, 1972. So, I want to know, since they are the only family who is staying in the house and we are the only outsider who purchased one flat, so, in the name of majority they can do whatever they want to do. What are the options so that they don't imply their decisions on us in the name of majority. N.B. We purchased the flat in 2011 and after full and final amount, they denied the payment and we had to do a complaint at consumer forum for registration of the flat and we won the case and then registration was done. So, if they form the association, then it's a threat for us. please advise about the options.