The brothers - Monaswar Hazarika and Hemkanta Hazarika - were locked in a land dispute with the latter grabbing a piece of land belonging to the former.
The district Court had recently ordered Hemkanta to return the land to Monaswar, Jorhat Superintendent of Police Amanjeet Kaur said.
Hemkanta attacked Monaswar with a spear while he was sitting outside his house in Yelengmora Khutiaphutia village under Pulibor police station last evening and hacked him to death following which Monaswar's two sons, Ananta and Ajay, retaliated by hacking their uncle to death.
The injured were taken to Jorhat Medical College Hospital where Dondeswar was referred to Assam Medical College Hospital and he died on his way to Dibrugarh.
The condition of the three others injured was also stated to be critical.
Tension prevailed in the area and a high alert has been sounded with additional security personnel deployed to prevent further clashes between the two groups of the family, Kaur added.