Legal opinion reg. Purchase of Mortgage Flat

I want to purchase a Resell Flat in Behala, Kolkata. My question is that the Flat is in the name of the landlord. The promoter developed this flat to the landlord joint ancestors property /land and gave 3 flat among the 3 ancestors of the family members relationally they are brothers and sold rest of the other 5 flat to others. I got this information from the broker that one of the flat is being sold by one of qthe landlord's family member. But the said flat is mortgage in a nationalized bank and the owner and his wife and married daughter are passed away, left only divorcee son and nephew to become the jointly legal owner of the flat. Divorcee son is presently residing in this flat. The mutation paper in the name of the late flat owner and his divorcee son joint name. Please advise is the flat legally safe to puchase and how do I move to execute the registration. Son is asking for token advance for my acceptance to purchase the flat. Regards. Sanjib Majumdar Kolkata Email: [deleted]