If you're looking to buy a cooperative society flat from a legal heir, and the original owner passed away without children, their brother's son (the nephew) may be the rightful seller. Ensure you have the surviving member certificate and a relinquishment deed from any other potential heirs, like the sister, in favour of the seller. While these documents, along with a legal opinion, should suffice, it's crucial to verify the clear and marketable title before proceeding with the purchase.