Regarding Fraud Done By Mother Related To Property

Dear Sir/Madam, I am Chaitanya from Hyderabad. I was married 2 years before. I have 2 lands and house. I have one married sister. My father died in 2012 in hospital. For my father's surgery in that hospital I paid 5 lakhs. One land is on my mother's name and another land is on my father's name. House is on my father's name. My mother sold one land for 2.5 lakhs for clearing my father's debts. Before my marriage my mother promised to my wife's parents that's that lands and house property belongs to my son. Now after 2 years of marriage my sister is asking 50% share in land and house property. As my mother promised is not happening now and my mother left home with those documents with her in my absence. Kindly guide me how to proceed as my mother done fraud and cheating with me. Kindly suggest.