Legality of unregistered will followed by a registered will

Hi sir/ Madam, This query is regarding the legality of a unregistered will followed by a registered will. Property Details: My father has purchased a land of extent 3456 sqft of which it was registered as two entities of each 1728 sqft built individual houses with joint walls for my father and mother during the year 1990. This dispute is related to the part which my mother owns . Family pedigree: My parents had three children , my sister being the elder one , myself an my younger brother Will History: My mother has executed a registered deed for her belonged portion during the year 2019 in the name of my sister without anyone's knowledge. Later we got to know from my mother that this ha been executed in the state of mind where she was physically and mentally ill due to her health conditions . This has been executed at home in presence of sub registrar . In the later days when she believed that the purpose of registering will deed in name of her daughter where she believed that she would take care of her during her last days was not happening once she registered the will deed. Followed by she requested her husband to prepare a will notary during the year 2023 mentioning the cancellation of the previous registered will deed and re registering will in favour of her hsuband which was not registered due to worsening of her health , unable to visit the registrar office and no available slots for home visit during the period. During this course it was her sons and her husband who took care of all the medical expenses and personal care during her treatment and last days before she passed away. After her departure my sister with her husband has occupied the property mentioned above without anyones notice followed by which my father has launched a petition in front of district magistrate requesting vacation and cancellation of the will deed thereby granting him rights over the property under code of civil procedure 26. My father is of age 78 years with underlying medical conditions due to which he finds difficult to walk / climb stairs to attend the court hearings, where my sister and their advocate have captured his day to day activities for potraying him as physically fit . Now i am requesting to help me understand the outcomes of this dispute and how to handle this further .