The incident took place at Rampurwa village under, Sujauli police station limits, a senior police officer said.
Some people in the village were talking when suddenly Sunil (28) attacked Pintu (25), leaving him dead on the spot, said Kamlesh Dixit, additional superintendent of police, Bahraich.
Sunil also attacked his sister with a sickle, but somehow she escaped, the ASP said.
Sunil has been arrested, Dixit said, adding the body has been sent for post-mortem.