According to a senior police official, the accused had allegedly abducted the victim's sister and had been staying with her for the past one year.
The incident happened on Saturday when the couple got into a brawl and the minor girl ran away to her parents' house in Vijay Vihar.
"As per the family members of the girl, the accused reached their house with a gun and threatened them to accept their relationship failing which he will shoot everyone in the family. The girl's brother refused and got into an argument with him following which the accused shot him in his stomach," said the official.
"A case has been registered under relevant sections and we are searching the accused. Further probe in the case is on," the official added.