The girl had run away from her house Saturday night and was molested by an off-duty conductor in a moving bus Saturday night around 11 pm at Barakhamba Road.
"We have arrested her brother Nazakat Ali, 19, for raping the girl. He was arrested from Khayala area in west Delhi," Additional Commissioner of Police V. Renganathan told IANS.
"The accused had sexually abused his younger sister six months ago and had been been molesting her continuously," officer said.
This came to light when police recorded her statement in the molestation case.
Police said a Delhi Transport Corporation (DTC) bus on the Khayala-Jal Vihar route heading towards Lajpat Nagar stopped at a picket near Mandi House when the driver asked for directions.
When the police officers spotted the girl crying, they asked her why and she told them an off-duty bus conductor in the bus first passed lewd comments at her and when she protested, beat her up. The bus conductor was arrested from the spot.
Police said the girl's father was living with his third wife and they stayed in a joint family.