The incident came to light when the girl was taken to hospital following complaints of regular stomach pain and the doctors found her pregnant, they said.
The stepfather and the landlord were arrested after a complaint was registered in this regard under the Protection of Children from Sexual Offences (POCSO) Act and several other sections of IPC.
"The victim was complaining of stomach pain for the past few days and had also stopped going to school because of that. She was taken to a hospital by her mother on Tuesday where doctors performed the necessary tests which revealed that she was pregnant.
After listening to her daughter, the mother approached police and lodged an FIR against the accused.
"The victim's mother who works at a thread-manufacturing unit, married the accused after her husband died some few years ago. From him, she has two sons," the officer added.
The accused were presented before a magistrate today and sent to Tihar Jail.