Succession Certificate

We are facing problem of getting a succession certificate as no proper guidance or anything the known advocates giving us. everyone saying its a complicated case. please guide us. My uncle have died in a massive car accident along with his wife and his one only unmarried son just 6months ago in same date and hospital after admission. We have PM report, Death certificates, Cremation Certificates, Hospital reports, Legal heir certificate of my uncle and brother, all 3 Aadhaar card of deceased persons. My uncle left 2 brothers(including my father) and two sisters. Uncle only son, i.e my brother was unmarried. My aunt has her mother who is 80yrs old alive. Now, who is the successors. Someone saying uncle brother & sister is the successors of your uncle & brother and aunt mother is the successors of your aunt. And someone saying aunt mother is the successors as my uncle died on that same day first at 10 am, then brother died at 10:30 am then aunt died 12 pm. Please guide us.