How can a registered gift deed be canceled?

My father had a property of 15000 sq feet and this was given by his father to him through a registered will and my grandfather got this through partition from his father. My father gifted the same total property to my mother through a registered gift deed in the year 2002. Later in 2004 they took my father and mother and cancelled gift deed in sub register office and did sale deeds for 10000 sq feet. Both my father and mother have signed the cancelled gift deed and did sale deeds on the same day and same minute. Can a gift deed be cancelled in sub register office or the cancellation have to be done in court and by the order of the court Both my parents are expired, as kids can we take legal action against the sale deeds and persons If we file a case will the court accept our case and does the case gain merits