The family of the 22-year-old woman had objected to the match so the couple ran away and came to Multan High Court in central Pakistan several days later for a civil ceremony, an official told AFP.
"The girl's brother had got information (the couple) were coming to court so he waited for them on the premises, where he shot them,"Rao Tariq, head of the local police station, told AFP.
The brother had already turned himself in to police, Tariq said.
Senior police official Ashfaq Gujjar said the girl died on the spot while the boy succumbed to his injuries in hospital.
"The lawyer who was with them was also injured and is undergoing treatment," he added.
Hundreds of women are killed by their relatives each year after allegedly bringing shame on their families in the deeply conservative Muslim country.
Under previous legislation the culprits -- usually men -- could get away with the killings after being pardoned by members of their own family.
But in July last year the high-profile murder of social media star Qandeel Baloch, whose brother confessed to the killing, reignited calls for reform.
In October parliament passed a law aimed at removing the ability to forgive "honour" killers. But critics contend some loopholes still exist.
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