Partition of self acquired property

My parents have a single plot on which a two storied building has been constructed. Ground floor is occupied by my parents and the first floor was rented out. We are three daughters and all are married. The said property is self acquired by my parents and is in the name of my mother. My mother had a pensionable job & father does not get any pension. My mother's pension and the rental income were the only source of income for my parents in their old age. One of my sisters (2nd) along with her husband and children moved to the first floor of our parents home in 2006 promising to pay rent, however they stopped paying the rent after a few months citing some financial problems. Inspite of repeatedly asking for the rent, they have not paid & also do not take care of any of my parents needs financially or otherwise. Iam taking care of my parents needs, I visit them every weekend and also support them financially for maintenance of the house , medical and other expenses. My youngest sister is a homemaker and stays separately with her husband and children. Now the 2nd sister & her husband who are staying in the first floor are forcing my parents to write a will and divide the property into two - first floor for them (already occupied by them) & ground floor for the 3rd daughter. They are pressurising my parents to not give any share to eldest daughter since she has a Govt job. My parents are against this and want to distribute the property equally among the 3 daughters. My parents are old and sick and are getting threatened by my sister and brother-in-law. I want to help them in making a will as per my parents wish and what is the legal way to deal with this situation. As the building is consisting of Two portions only, how can it be divided ? Shouldn't my parents also get a share ? Kindly advice.