A 15-year-old boy is being held at Ukwala police station in Ugenya for allegedly defiling his eight-month-old niece on Friday.
The infant had been left under the care of the Class Eight boy in Ogarama village by her mother, who had gone to the market.
Assistant Chief William Lala said the woman returned and found her daughter bleeding from her private parts.
The baby was taken to a dispensary before being referred to Ukwala sub-county hospital.
The woman later reported the matter and police initiated a search for her teenage brother, who was already on the run.
Police boss Patrick Lumumba said they arrested the boy and that he would be arraigned tomorrow.