Gift deed has been revoked unilaterally

My father in law has 2 daughters and no sons. he has constructed 2 independent houses. ( a and b ).he has executed gift deed one independent house (a) in favour of his younger daughter in 2009 and she has subsequently sold that house in march 2016. my father in law has executed gift deed in respect of the independent house (b) in favour of his elder daughter (my wife) in july 2016. since 2016 his elder daughter (my wife) is abroad on work. she has allowed her parents to stay in her gifted house as long as they live and has given an affidavit (mutual understanding) to this effect. my father in law is a paralysis patient having lost mobility of one leg, one hand,speech and vision in one eye. in November 2018 he was admitted in a hospital in a critical condition and his body has become totally immobile and is bed ridden.For the past 5 months his younger daughter visited her father daily and taking advantage of the absence of her elder sister and her father's physical disability she has narrated all types of gossips, viz. ELDER DAUGHTER will sell her house, she will throw you out etc. Ultimately she made my father in law to REVOKE the gift deed in JULY 2019 which was signed UNILATERALLY as my wife is abroad. He can't speak anything. Now that younger daughter has got that HOUSE 'B' (which was gifted to us earlier) transferred to in her name as GIFT DEED. Sir kindly inform us the remedies available for us to challenge and get this 1. REVOKATION order cancelled and 2. Stop in executing the gift deed in younger daughter's name.