Police are holding a man suspected to have killed his mother and seven-year-old brother in the Kamulu area, in Ruai, Nairobi County.
David Kimani is said to have used a stone to kill his mother and brother after hitting them on the head.
The deceased Peninah Njoki Kimani aged 43 and her son Maina Karibu died on the spot.
The incident happened on Tuesday morning.
The bizarre incident also saw a neighbour who had come to help after hearing screams hit.
She was seriously wounded and she is currently in hospital.
Nairobi police boss James Mugera confirmed the incident and said the suspect is being held awaiting legal procedures.
“The man will be among others undergo medical tests before he is prosecuted with murder. We are investigating it,” he told crime journalist Cyrus Ombati.
Domestic quarrels have been linked to the harrowing incident even as the probe continues to establish more details.