Adoption child

Dear Sir/madam, My father has 2 elder brothers, 1 younger brother & 1 sister.1st elder brother get one Male child from the 2nd elder brother in 1 month like a stepchild(Valarpu magan), And he is 36 years old. he doesn't have an adoption agreement or any legal documents. Only he has his name on the school certificate and Aahdar card 1st elder brother is the father's name. In the birth certificate, 2nd elder brother is the father's name. Right now, My father 1 elder brother passed away in 1996.his stepson now stays with my father's 2nd elder brother and My father 1 elder brother's wife also passed away in 2021. we have our own family property of our Grandfather. Now, 1st Elder brother's stepchild asking for one separate equal share. 1. According to law, stepson will get a share in the grandfather's property? 2. During the presence of 1st elder brother's wife. she got already successor certificate (only one Successor is 1st elder brother's wife)