cancel of deed of gift (settlement)

Our father purchased house in the year December, 1984 and registered on our father's name. Then, our father executed and registered a Deed of Gift (Settlement) of the same house favoring our younger sister in the year i.e., February, 2010, which is unconditional. Actually, the property is acquired by our father and he expired in the year 2012. Our mother is alive as on today, she is in good health, and she wants to settle the house among three sisters. We are three sisters and we have equal share of that house. Without knowing any of family members that house was registered to my younger sister. Now, after 4 years, can our mother cancel the said Deed legally? If so, can she succeed in that case? Or shall we proceed to go ahead with the partition suit against our younger sister? In view of the above, please kindly forward your valuable and genuine advice and guide us in the right path to proceed further. Thanking you, Yours faithfully, Sarala Hyderabad. .