Grand father extra marital affair woman claiming ancestral property

Hi sir, We are from Hindu religion. My grand father married to my grand mother approximately in 1960. My father born in 1961. When my father is 8 years old my grand father brought a women to home which he already had extra marital affair. Unfortunately she is niece to my grand father's mother. So they did not oppose for that. Since my grand mother is innocent she did not complaint in police station. Time passes the another women got four daughters. Approximately in 1985 my grand mother expired. In 2003 my grand father expired and in 2014 the other women expired. Now the property disputes are arrived and the other women daughters are making lot of issues with property. The property sourced from my father's grand father. After his death my father and grand father got passbooks based on half and half on hole property but physically is not separated till now. After my grand father death the other women transferred the half property on her name. Kindly suggest legal proceedings