Fed up of harassment by your stepmother & her claims on father's money & property?

Dear Sir/Madam, My name is (undisclosed) aged 35 years old and I’m an NRI living outside the country from last 8 years. I am writing this to seek further right legal guidance to take necessary action against my stepmother. My father (67yrs) is a retired Range officer of the forest. We are three brothers and sisters, elder one is a sister (37yr) who is married and living with her husband, I’m married too and reside outside the country, the third one is my younger sister (27yr) who is unmarried. My mother was a housewife who expired in 1997. After 3year of my mother's expiration, in 2000 my father got married to my mother's younger sister (masi), and marrying my dad was her second marriage. With my stepmother we have one brother(21yr) too who born in 2000 itself (in the same year of marriage). My stepmother is horrible and has a criminal mind. From the first year of marriage, she started fighting with us over small things and used to abuse us, even though she sometimes beat our younger sister, at that time she was only 4-5 years old. The toucher level gets worse year over year and is unmiserable. We all clearly know that her intention is that she does not want us to be around so that it will be easy for her to usurp the entire property and assets of my father and late mother. She also tortures my father that she will file a case of domestic violence against him if he pokes his nose in between and can't escape it. The present situation is that my father is in his 60’s age and is not fully physically and mentally capable to fight alone with her legally. My father lives with my younger sister who is taking care of him in every way, both live in the same house (lower floor) and my stepmother step brother lives on the upper floor who does not even offer a glass of water to my father out of courtesy, infact on top of that sometimes they used to beat my father and sister. Her intention is only around money, or to be precise she married to my father for money/property. My question is that the house we are living in (which is under my stepmother's name) was bought with the money that got after selling a plot that was under my late mother's name [that plot was gifted to her by her mother (nani)]. 1. Can we (my father and three children of first mother) have rights to claim on house? 2. Does my father have any legal right to pass the ownership of the house to us? 3. Can she still claim the house as it's under her name and she has a child? 4. What if my father makes a WILL? Please help, my father and younger sister need legal assistance. That woman had put our lives on stake. Thanks