Registration issuse with builder

My grandmother had two daughters. one, ran away from home because of some love affairs & she got married  and since then she dont have any connection with our family and finally she died
28 years before And she has 2 sons.And the second is my mother. My grandmother owns a flat and registered under her name and she had written a will papper on my mother's
 name but its not registerd . And the property undergone  re-development. During that period my grandmother made the agreement  with the builder mutually
ie( with her name & my mother's name). After few years my grandmother got expired. Now we are planning to register the flat on my mother's name,  the  builder is asking for the NOC from
 those two sons.As our family dont have any connection with them, we are unable to recive the NOC from them. Mean time we had given a public notice in the local newspapers seeking any
individuals with a claim or interest , or demand on the same property should revert us with  the supporting  documentswithin 15 days. But we haven't recived any claims.
 Kindly suggest me the next procedure inorder to register the same on my mother's name.