How to sell a property if there is a missing ownership

Need Solution - WE have purchase one flat by agreement to sell from Person. But they are not Fully owner of that flat because from builder they also took the flat by agreement to sell on their name. But afterward Builder directly made SALE DEED on My mothers name without taking consent fromthe Person. Now we want to sell that property but problem occurs to sell as property ownership is not define yet. Still that persons existence seen in Search Report. And current scenario is that The person who taken flat by agreement to sell is not exists. Now We have to taken consent for completion of our Sale. From Whom should we took Consent. Wife of that Person, OR Children of that Person or Wife of Death of one of the children or all of them?