Succession certificate or legal heirship certificate

This query is for my relative's son and daughter. Their father had expired 8 years ago and the flat was in mother's name. Mother had nominated son and daughter as nominees but made no will. Now mother expired in April 2019 and they applied the society in Kandivali, Mumbai to transfer the flat in their name. The society is asking to provide succession certificate or legal heirship certificate for transferring share certificate in their name. Else, they will make a note on the certificate that the brother and son can not sell the flat after transfer because they were nominees. questions: 1. Is this stand of society correct? 2. What is the total cost (your legal fee and court fee) for getting the succession certificate. 3. What is the total cost (your legal fee and court fee) for getting a legal heirship certificate? This question has been asked assuming there is no cost involved to get the reply online. If there is any cost to be paid for answer, please do not reply as I am asking this onsomeone's behalf.