Pawan alias Golu (20), was arrested yesterday evening from Bapu Park near D- Block, Mangol Puri after police got a tip off that he would come to the spot.
On Friday, a PCR call was received at around 6 PM that a youth had been stabbed in the C-Block area and admitted in Sanjay Gandhi Memorial Hospital.
"A police team rushed to the spot and found one Satish, who informed them that his cousin Gaurav alias Devender was having an affair with a girl in his neighborhood and the brother of that girl had warned Gaurav to keep away from his sister.
Following his death, a case was registered and investigation was taken up, the official said.
When Pawan was nabbed, the weapon of offence, a knife was also recovered on his instance, he said.
During interrogation, the accused allegedly told police that he had killed Gaurav because he was not happy with the victim's relationship with his sister, police said.