To ratify any agreement one has to own the subject property. In absence of a will, no one is the owner (at present), hence can not ratify it. First step is to own the subject property through court. You have not mentioned
1) whether the property is ancestral or self owned/purchased by your father and
2) it has single owner( your father) or jointly owned by your mother.
Contact a local lawyer, file a succession suit in the district court of the area where the property is situated. Your presence is required in court case, Aadhaar is not mandatory. Documents required for the suit are original registered property documents, copy of death certificate of your father and list of legal claimants & their details along with signature.You will also be required to put advertisement in the local daily (newspaper). You will be required to pay stamp duty in the court itself as decided by the court. Through court the property will be transferred in the name of your mother and two children. this whole procedure might take 6 months approximately.
Once the property is transferred in your name, you all can ratify the agreement made & agreed upon between your father and the builder.