A 13-year-old boy was shielded by his mother and sisters on the pretext that he was the 'only son' in the family after he raped and strangled his sister to death after watching a porn video on a mobile phone in the Central Indian state of Madhya Pradesh a few months ago, police said.

The shocking incident happened in a village in Rewa district on April 24 and it took around three months for the police to detain the culprits since the mother and the sisters tried to cover up his crime.

"A nine-year-old girl was raped and strangled to death under Jawa police station limits on April 24, after which the police had filed a case and started a probe. The victim's body was found in the courtyard of her house where she was sleeping at the time of the incident," Superintendent of Police Vivek Singh was quoted as saying by Indian news channel NDTV.

The 13-year-old teen's mother and sisters, aged 17 and 18, admitted that they tried to hush up the incident and save the culprit during the intense investigation.

During the questioning, the boy said he slept beside the victim on the night when the crime was committed.

The teen admitted that he raped his sister after watching a porn video on his phone.

Police said the boy strangled the victim after she said she would inform the father about the incident.

The boy informed their mother about the act and once he found that the victim was still alive he strangled her to death.

Singh told the news channel that the two sisters, who tried to cover up the crime, changed spots on the bed to mislead the investigation.

They even tried to deceive the cops by saying that the minor died after she was  bitten by a poisonous insect.

They even took the body of the victim to a local doctor but after being refused by him they were forced to take her to the government hospital.

The doctor in the medical facility declared the cause of the death to be rape and murder.

Police said technical evidence was collected and  50 people were interrogated when cops found family members were repeatedly changing their statements.

Police said the family members admitted to their crime following intense interrogation.