The accused identified as Yamuna (24) and her husband Rajesh, residents of Raghubir Nagar, have been arrested, they said.
A day after the toddler went missing from his home, police found his body in a flooded drain in Punjabi Bagh.
According to police, Yamuna felt that her mother loved her sister”s son more than her daughter, due to which she hatched the plan to kill the boy.
Ten teams were formed and they started tracing the missing child, a senior police officer said.
Police analysed the CCTV footage and saw Yamuna and Rajesh taking the boy towards Ganda Nala, Punjabi Bagh, Deputy Commissioner of Police (west) Urvija Goel said.
Yamuna was held from Subhash Nagar and upon interrogation, revealed that she and Rajesh had kidnapped and killed the child, the DCP said.
The accused took the toddler to Punjabi Bagh drain, strangulated him and to make sure that he was dead, drowned him in the drain, police said.