A man killed his sister allegedly in the name of 'honour' in Lahore's Satto Katla area on Wednesday, police said.
The man opened fire on his sister after having an argument in which he accused her of having an "immoral character", DawnNews quoted the police as saying.
Police took the girl's body into custody after residents of the area reported the incident. They also collected evidence from the site of the murder and recorded statements of eyewitnesses.
According to the police, the suspect fled the scene after killing his sister.
The victim's body was shifted to Jinnah Hospital for postmortem. A case has yet to be registered.
Honour killings continue despite new law
Scores of young women in Pakistan are still being murdered by relatives for bringing shame on their family, a year since new laws came into force aimed at stemming the flow of 'honour killings'.
The shocking murder of social media star Qandeel Baloch, allegedly by her brother last July, turned the spotlight on an epidemic of murders in the name of honour and sparked a fresh push to close loopholes allowing the killers to walk free.
Last year in October, the joint sitting of both houses of parliament finally passed two key pro-women bills that had been pending assent for a long time.
The move at that time was cautiously hailed by women's rights activists. More than a year on, however, lawyers and activists say honour killings are still occurring at an alarming pace.
At least 280 such murders were recorded by the independent Human Rights Commission of Pakistan from October 2016 to June of this year ─ a figure believed to be underestimated and incomplete.
The legislation mandates life imprisonment for honour killings, but whether a murder can be defined as a crime of honour is left to the judge's discretion.
That means the culprits can simply claim another motive and still be pardoned, according to Dr Farzana Bari, a widely respected activist and head of the Gender Studies Department at Islamabad's Quaid-i-Azam University.