The teen, a native of Muzaffarpur in Bihar, used to sell vegetables in the area (Representational)

A teen was arrested for allegedly killing his sister-in-law and her minor son after she resisted his attempts to sexually assault her in central Delhi's Prem Nagar area, police said Saturday.

The teen, a native of Muzaffarpur district in Bihar, used to sell vegetables in the area. He was staying with his sister-in-law's family for the past 20 days, they said.

On Thursday, the bodies of the 26-year-old woman and her four-year-old son were found at their rented accommodation.

The woman's throat was slit with a sharp object and she was lying in a pool of blood. Her son was found hanging from a ceiling fan, police said.

During investigation, it was found that the teen was the last person who visited their room, they said.

While police scanned the mobile phone of the dead woman, they found a selfie which was taken by the teen Thursday afternoon just before the incident, police said.

"Police arrested the accused person after they found some minor injuries on his body, including teeth bites on the legs, scratches on face, neck and the back," Mandeep Singh Randhawa, Deputy Commissioner of Police (Central), said.

During interrogation, the teen revealed that on the day of the incident he tried to assault the woman which she resisted strongly, he said.

In her resistance, he received minor injuries on his face, neck and other parts of the body, following which he hit the woman with a stone, leaving her unconscious, the DCP said.

In the meantime, the child woke up and told the accused that he will tell his father about the incident.

He strangled the child and while he was trying to hang him from a ceiling fan, the woman regained consciousness. She bit the accused on the leg to save her son, Mr Randhawa said.

The accused hanged the child after slitting the throat of the woman with a knife, after which he washed the blood stains in the washroom and left the house, the DCP said.

After the incident, the teen wandered about for two to three hours, the officer said.

A pendant of the accused, a stone, a knife and a soap used for washing blood stains were recovered, police said.