About my home issue. I am paying the taxes from past 2 years.

Hi sir/mam My grandfather have 2 sons and one daughter. And he have one house. Before expires he wrote an agreement that regards after his deaths this house is belongs to his two grandsons(elder sons sons) that means me and my brother. Until he expires he can change his agreement. After some years After the registration he got expired. That property changed into our property. After 3 years his younger son is asking the share in the property. That is only one house. We can't share that one house. Is there any rights that he can get. He have one son and and one daughter. There is no issue with my father's sister. But younger son is asking the share. In the agreement my grand father clearly mentioned that I am not interested in giving the property share to my younger son because he is well settled. But he is telling that with out his signature this agreement in not valid. Is that right. Does my grand father need his permission to register his home on some others name. I am hindhu.