Mother and Daughter after husbands illness

My name is Gaurav and I am From Jharkhand. My father is suffering from mental disorder from year 2017 . During that time my sister and mother secretly bought a flat worth 50 lacs in my sisters name without keeping me in notice later I came to know about it through some other means and then they said she would transfer it as there was some Pancard issue. She denied giving the flat post her marriage in Dec 2020. After my marraige my sister manipulated my mother and get things done. My father in getting retired this month and now she is trying to manipulate mother for the all settlement amount which is to come in near future . Also she says daughters have equal rights etc but not ready to take responsibility of my ill father and old mother. There is a small joint land which my grandfather gave to my father and his mother their brothers together. She is claiming that as well and other thgs. Need your suggestions and legal advice that what can a son do if the mother is supporting daughter and not son when father is in severe mental condition and cannot think goodwill of his son