The boy is reported to have found his elder brother using his tooth brush, leading to a violent confrontation.
“They were fighting over a tooth brush when he stabbed his brother,” a police officer said, quoting a witness.
The boy died on the spot following the attack that occurred in Dandora, a densely populated suburb estate in the capital.
Nairobi Provincial Police chief Moses Nyakwama told Capital FM News that the boy was being held to facilitate investigations over the incident.
“He is in custody and after investigations, he will appear in court to face murder,” the police chief said.
Several witnesses have recorded statements with the police over the matter.
Officers had recovered a kitchen knife used by the boy to stab his elder brother who was in form three.
Meanwhile, four suspected gangsters were shot dead and another lynched by members of the public near Lenana School in Nairobi.
Police said the suspects had allegedly stolen Sh50,000 from an M-Pesa shop at Ngando centre in Dagoretti when they were confronted.
He said his officers also managed to recover the money and two pistols from the slain suspects.
Police have said the slain gunmen were on their wanted list for terrorising investors and residents in the area and were planning robberies at the Nairobi ASK showground.