Please help me urgently :(

Dear sir

          I want help from you, my father has written a  gift deed to my sister on her marriage occasion worth rs 50,00,000 by giving right to sell to my sister children (male/female) but my father's intention is, to write only the right to get money from it but not to sell.But they are trying to sell it now.

             Please let me know, whether my father can amend the gift deed now as he wish? Is there any time limit to get such amendment. Please tell me. Exactly 1 year is expiring on this month. 

                Thanks in advance.

                 B.SAI KRISHNA