legal rights of mother on House owned by father

Dear Sir My parents' relations are quite strained since their marriage. we are 4 sisters out of which 3 are married now. My father owns a house in Delhi which was given to him by my grandmother. Now we fear that after the marriage of our 4th sister he would give this house to his deceased elder brother's wife/ son/sons or as he has threatened few times he might sell off this house and leave with all the proceedings leaving my mother stranded. My father's brother's wife and her children have suddenly developed interest in him after so many years of non communication. They are constantly in touch with my father as they know they can make him against my mother more and get this house in their name. Registry of the house is with my father and we have not seen it for quite some time. We dont even know if he might have already transferred this property to his nephews or someone else. Pls suggest what are the legal rights of my mother and what can be done in precaution so that the situation can be in favor of my mother or to stop my father from transferring the property or from selling off the property. My mother is a heart patient and is presently staying with me due to her ill health since last 6 months but keeps visiting father's house for a week in a month. Waiting for your suggestions eagerly. Kind Regards Deepti Kar