Heritance, ancestral, Will

My grandfather built house before his death my father got the house with no objection from his sister's. House was on my father name as part revenue And house tax records. We are four sister's only. Before my father died he made will to give house to my mother and if she or him die then house should go to two younger sisters. Nothing to me and my elder sister although we always had good relationship. My father died. Now house is on my mother's name in records of Revenue and house taxes records. My questions are: 1) Can my father will house that he did construct as it was made by my grandfather? 2) Is his will valid and enforceable even when the house is now under my mother's name? My mother has not made any known will so far. 3) Is my father's will has 12 year period when not contested can become permanent? Even if my mother makes no will his will be valid after death of my mother? What's best course of action for me to take so that I do not loose my rights and share of the property. Thank You. Sneha Lata