Domestic violence, Divorce and property claim after 30 years of marriage (non registered)

My mother being the only child inherited her parental house after her marriage (in 1985), where my parents and i and my elder brother has lived for last 30 years, at least. My father had spent a huge amout of money in expanding the house. My mother was toutured both physically and mentally by my father and his elder brother and sister in-law in many occassions over past 10-12 years at least, the torture has increased after my brother got married 5 months ago. Now my sister in-law has started abusing my mother and is threatening my mother, brother and me (I live in some other state for last 5 years and go home during vacations once a year) to charge with domestic violence. My mothers wants to apply for a divorce/ separation and she wants to claim the house since my mother is the next of kin after her parents death (they didn't make any will for the house). My father is threatening her that he has spent >10Lacs on the house, so he can contest for the house claim. My father is a retired State Govt. employee who gets around 20k per month as pension. If my mother files for a divorce how much she can ask for maintenance? My mother being a housewife, doesn't have much savings so without the house/inherited money, she cannot fight in court. Thank you.