The accused committed the crime on April 7, 2018 at their home in Heshima, Nakuru County.
According to the investigating officer, the boy arrived at his mother's home at around 1 pm and demanded to be given food.
When his siblings and his 12-year-old sister told him that there was no food for him to eat, he got violent and started beating them.
"He directed his two siblings to go to the next room and took the complainant [12-year-old sister] to his mother's room.
"He physically assaulted her, tied her mouth with a piece of cloth and went ahead to defile her on his mother's bed," the police report read in part.
The sister reported the matter to Bahati Police Station and the teenager was arrested.
A medical report also indicated that the victim had a broken hymen despite the teenager's denial that he had assaulted his sister.
His mother, the complainant and the two siblings all gave testimonies against the convicted teenager.
In his defense, the accused claimed to have been framed by his family due to his differences with his mother.
"I demanded to know who my real father was from my mother. She had refused to respond to the question. I was chased from home in 2013 and only returned in 2017," he narrated.
He now has 14 days to appeal the court's decision.