Builder Contract Only After Signing of POA

Hi, We have an ancestral property which belong to my grandmother. She has written the property to her grandchildren and made her children as legal guardians of the property. Now my grandmother's children want to demolish the property and build a new multi-story apartment for which they have appointed a builder. The builder wants everyone of us (grandmother's children + the grandchildren) to sign the POA on the builder's name to proceed with the work. I had asked for the contract that is between us and the builder BEFORE I can sign on the POA to the builder. For this the builder says that the POA must be signed first by all and then only the contract between the builder and us can be made. This does not make any sense to me because how can I sign a POA to the builder without even a contractual agreement on what the new construction would be, cost, timelines, etc.? I need someone to please let me know how does this work as per Indian Law. Thanks and Regards, Wesley