The incident was reported from Mongolpuri on Tuesday and both the accused -- Manoj (22) and the 16-year-old brother of the girl -- were apprehended yesterday.
Pradeep, who was in love with the teenager's sister, and his friend Tinku were on way to buy a TV remote when the duo allegedly opened fire at them.
While Tinku suffered bullet injuries and died on the spot, Pradeep escaped with minor injuries.
"Local residents were very angry as the deceased was only brother of two sisters and his parents had already died. A lot of pressure was mounted on police by locals.
"Pradeep was very afraid and initially, he did not not co-operate with us but later he helped us," DCP (Outer) B S Jaiswal said.
The youth told police that he was in love with the sister of the teenager but the latter was very angry at it. The girl had told Pradeep that her brother was allegedly making plans to harm him, but he ignored her advice, Jaiswal said.
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Manoj was arrested yesterday on a tip-off. Later the teenager was also apprehended.
"The brother was very angry with Pradeep. The duo had warned Pradeep several times but he had paid no heed. So they hatched a conspiracy to eliminate Pradeep. To complete his mission, he procured a country made pistol from Baghpat," the DCP said.