The 26-year-old Kajal was arrested after post-mortem report revealed that her stepson Prince was murdered, said a police officer.
According to police, Indal Paswan, a native of Bihar, worked as a scrap dealer and lived with his wifeKajal, two sons and a brother in Vijay Vihar here.
Prince was found dead yesterday.
The accused told others that his health had deteriorated the previous night and he had died due to illness.
Kajal, along with her brother-in-law and neighbours, took body of Prince to a local hospital where the employees suspected the death to be unnatural and informed police.
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Kajal was interrogated and she admitted to killing Prince, a senior police official said.
Police said, Paswan had married Kajal five years ago after death of his first wife. Prince was then 3 years old.
He also has a son with Kajal.
She also said Indal had married her without telling about Prince, the officer said, adding that she allegedly used to abuse her stepson.
She also started believing that Prince will take revenge with her when he will grow up, as warned by her husband and others, he said, adding that Paswan had gone to his native place when the incident occurred.