Question on Revocation of gift deed and the necessity of a Decree

If the following facts are favorable for cancellation of a gift deed already executed. That just prior to the date of execution of the gift deed the donor had a history of hospital admission because of cerebral attack and continued treatment under different specialized doctors during follow-up periods when diagnosed with cancer and further admitted in hospital but in vain due to his worse condition as was discharged for peaceful die at home having limited days to go. That the donee having being informed of such condition of her father rushed to her father for the first time since her marriage in 1996 to take care of her father when her elder sister, the constant companion of her sick parents for the last 20 years left her father (absolutely in an infirm state of condition) in the custody of her sister (donee) on good faith. That the gift deed was executed by commissioning at home within the next 5 days having the donor under the absolute custody of the donee when donee flew away to Lucknow taking with her parents without any intimation to her elder sister. Elder sister immediately registered several complaints with local police. That the certified copy of the gift deed exhibits the following; a) The donor executed the deed with his LTI. b) The donee's husband attested the donor's LTI without there being written ' Read over and explained...' c) The donee's husband also signed as one of the two witnesses. Lastly, the donor had died under the custody of the donee at Lucknow within a week after reaching there. QUESTIONS 1) If cancellation of the alleged deed is possible? 2) If cancelled, whether changing the title of absolute ownership is possible at the registration office and the mutation record also? 3) If a decree is mandatory to get the changes as asked in serial (2) above?