Who will be legal heir

HI SIR, who will be the legal heir... pls advice.. My father had legally married wife and had 2 sons and a daughter and all are married and having kids. In addition to it, father had married 2nd wife NOT legally and have one boy. w.rt properties, there are properties which belongs to my ancestors which was automatically has come to father as he is the only son for my grand parents and some properties are bought by my father itself. After 15 years now, would like to know who are all will be the legal heir.. Is the 2nd wife as its not legally married but i believe the 2nd wife son - father name in his birth certificate will be my father name.. and also i believe there will be some more proof related to schooling etc will be with them. But we believe father is not having contact with the 2nd wife for the last 10 years. Thus we are all the wards of the legal wedded 1st wife thus, would like to know who will be legal heir of the rights and properties in this regard. Please advice as its has made us restless and confused and want to know whether to prepare or get any legal signed documents from 2nd wife and her kid to avoid any issues in future. Thanks.