According to Area Chief Rosemary Kathuku, the incident occurred immediately after the family returned from the morgue.
The deceased's body was later taken to Makindu Hospital mortuary while preparations for the other brother's funeral were still underway.
Police in Makueni have launched an investigation into the murder, which has left the local community in shock.
Reports from Citizen Digital indicate that the man allegedly killed his brother in Malivini village after a dispute arose over unspecified issues.