A 16-year-old boy tied up his cousin to bed and allegedly raped her in Gurgaon's Sector 51 area, police said on Saturday.
The incident happened on Thursday, but was reported to the police on Friday after the 15-year-old girl fainted in school and narrated it to her teacher.
The girl's mother has filed a complaint at the Woman's Police Station against the son of her sister-in-law, a Gurgaon police official said.
The woman had sent her daughter to her sister-in-law's home to help her with daily chores as she was not keeping well.
When the sister-in-law went to doctor leaving behind her son and niece, the boy groped his sister, tied her hand and legs to the bed and raped her, the official said.
The boy has been arrested and a case has been filed, he said, adding that further investigations are underway.