Hi
1) In general for adoption, it is the customs and rituals that are generally considered for adoption Hindu Adoption and Maintenance Act (HAMA) 1956 .
2) Till the year 2000, no registration of adoption was legally necessary and it was through custom that the adoption of children was done.
3) So your brother cannot now deny the adoption and claim the share from the biological father estates.
4) Also if the school leaving certificate, ration, aadhar card etc bear the name of your father''s brother as adoptive father, then your brother cannot claim the fact of adoption perse.
5) Also since your father is alive, your father can testify in the court that the brother was indeed adopted by his brother.
6) Also , you should be aware that law of limitation will be applicable in this case.
Right to sue for declaration relating to status of adoption arises when the adoption is denied.(Reddy Subbarao (Died) Per L.Rs. vs Nunne Venkataramanna And Ors. on 5 January, 2007, AP high court)
7) Your brother cannot now claim that he was not Adopted at all, as ideally if your brother is aged about 21 years, he should have contested the adoption on or before he reaching 25 years and not later than that.
Hope this information is useful.