Property division dispute among brothers

My Father has 8 siblings, 7 Brothers and 1 sister. In total they are 8 brothers and 1 Sister. Out of these one brother was adopted by his Uncle. There is an ancestral property, which is in form of different agricultural land. The same was divided by their father between 7 sons and there is a documented proof of that and also two witnesses had signed that. The partition was not registered. Now the land situations have changed and one of the brothers is not ready to accept the division made by their father. He wants land in all parts to be divided equally. Questions are: 1. How valid is a division signed by all sons and father in presence of two witnesses? 2. Can a son adopted by uncle claim right in his biological father’s property? 3. What are the possibility of case getting settled in favor of 6 sons who are okay to accept the division made by father in past? Thanks in advance for you time.