As per your statement above few things need to be clarified.
Was there a Site in your Grand Father's name. and on that site your father constructed a house and is registered in your grand mother's name after the death of your grand father . Once the property is registered in your grand mother's name, your grand mother has become an absolute owner of the property. Unless there is some written MEmorandum of understanding - registered between your mother/father and your grand mother regarding the rights of alienation which restricted your grand mother to sell the property in question which is being constructed by your father your grand mother can sell the property to anyone she wishes to in her capacity as absolute owner. If she has gone to sub registrar office and her photo graph appear in the VEndor column and her signature affixed in the Absolute Sale Deed then it is clear that the property is legally conveyed to the PUrchasers ( here UNCLE and her Cousin ). Difficult case unless you have some strong apprehension that there is a element of fraud which u can prove.
U can revert back to me with all the available documents with you. with the signatures of your grand mother and the documents etc then we can give a try after through scrutiny of the documents and depending on who is in possession of the property.. etc
U can revert back to me on firstname.lastname@example.org or on 9845285555 in Bangalore.
Mahadeva Rao G
ADVOCATE, 9845285555, BANGALORE.