On Monday, the police received information from a local hospital that a girl, who was bleeding profusely from her private parts, had been brought in, a senior officer said.
The girl told a counsellor that her elder brother forced himself on her while their parents were away, the police officer said.
The parents returned home in the evening to find the girl bleeding and rushed her to a hospital, he said.
The boy has been detained, he added.
Delhi Commission for Women (DCW) chief Swati Maliwal visited the victim on Wednesday.
“She sustained severe injuries and had to undergo a major operation. Currently, she is in a serious condition,” the DCW said in a statement.
“The girl’s parents are daily wage labourers and are very poor,” it added.
The DCW said it will support the girl’s treatment and file a case to seek compensation for her.
“Crimes against women and girls are on a rise in the country and concerted steps have to be taken by governments against them,” Maliwal said.
“The commission shall be assisting the survivor in all ways possible to recover from the trauma, Maliwal said.
According to Delhi Police data, more than two child rape cases are reported in the national capital every day. Till April 30, 282 cases of child rape were reported compared to 278 last year during the same period.
Last year, a total of 894 cases of child rape were reported, data showed.