The incident occurred last evening when Arvind (20) and Bablu (22) were cleaning the sceptic tank with the help of their mother Malti (48), said SP (Rural) Rakesh Kumar.
"Arvind was the first to enter the tank with the help of a ladder. He started asphyxiating due to the poisonous gas and fell down," the SP said.
Bablu tried pulling his brother out but he too inhaled the gas and fell unconscious.
By the time the gas escaped through a hole dug by locals the victims' condition had become critical.
They were rushed a government hospital where doctors declared them brought dead, the SP said.
The bodies have been sent for postmortem, police said.
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