Property purchased from Legal Heir after Father/Mother Death.

I'm planning to purchase property, here's scenario. 1) Owner (Seller Father) died in 1997, leaving behind wife and 4 childrens (I'm not sure if there's any more children) 2) Society did transfer on wife's name (Seller mother) and went for redevelopment, and new builder agreement executed on wife's name and society transfer certificate also done. 3) Wife (Seller mother) died recently in 2015/2016, leaving behind 4 children. 4) within mutually agreement between all brothers and sister, they did release deed on first brother name (seller) and society transfer is also done on first brother (seller) name. My question, 1) Can I execute sale deed or agreement for sale with Seller (do all brothers/sisters need to be confirming parties in sale document?) 2) How I can verify, there's no other legal heir present (I've checked there names on Ration card, Aadhar card, Nomination form mother submitted to society). 3) How to avoid complication if any more legal heir appear/claim after I purchased the flat. 4) Please let me know precaution and proper document to be executed. Note: I'm taking home loan. I would like to pay for consultant fees if required.