Family problems & custody of step sister

Hi,we are three sisters & a brother(me,my elder sister,my younger step sister & my younger brother),my mom expired when i was 13 yrs old within 3 months my dad remarried and got us a step mom.she is a monster she made our life miserable.but we kept quiet as our dad was with us.few years later she gave birth to a baby girl our step sister. Now my step sister is 8yrs old and we all are well settled in our lives me & my elder sister are married & my brother is a marine enginer. Now that she knows that dad is alone as we are settled in our lives & my brother is away from home due to his job..she is threatening my dad & wants him to transfer the property in her name. We also got to know the my step mom is a female with loose character and had married my father just for property.She has contacts with lots of people who are from police background as well as lawyers so she is trying to threaten my father by getting these people at home. Now she wants my dad to transfer all the property in her name and threatening my father the she would take away my step sister from him if he didn't transfer the property in her name. Please suggest me how can I help my dad & protect my sister from this female.She has also threatened my step sister that she will kill her if needed and nobody can do anything about it as she has contact with big people. Please help....