Superintendent of Police (City) Dinesh Tripathi said the incident took place in Karauda village under Madnapur police station, where Devesh died succumbing to the injuries.
Prima facie it seems to be a case of enmity, the official said.
"The deceased's brother had eloped with a girl almost one-and-half years ago and married her. Today's murder of the 12-year-old seems to be a case of revenge," Tripathi said.
A case of murder was registered against six persons while the body of the deceased was sent for post-mortem, he added.