How to file for Family certificate when there are 2 wives

Respected sir, 40 years back my parents r wedded and they blessed with two children. I am one of the children.35years back my father married another lady and they r blessed with one boy. Both wives r alive. My father has passed away very recently . We means first wife family spent a lot of money for curing my fathers health condition.second family they did come even to see my fathers dead body.Issue is second wife & her son r not co.operating to get family certificate to claim bills. My step brother he is nrot responding to good r bad who is staying in abroad. He just cut contacts with our people. Govt compelling my step brothers signature or affidavit from him.but he is not responding.rejecting of certification is happening in the absence of his name. What is the way to get family certificate in case he & his mother is not responding.