The girl, identified as Nazeeran, was speaking with her husband-to-be Shahid in Nayi Wahi village in the suburbs of Ghotki town when her uncle spotted them.
The uncle opened fire on them out of anger, Express News reported.
According to police, the victims were each other's cousins and the incident was a case of 'honour killing'.
Two suspects, both of whom happen to be the girl's uncles, have been arrested so far.
Honour killings are pandemic in Pakistan, and women are its predominant victims.
An angry Pakistani youth allegedly shot dead his sister and her husband in Rawalpindi last month for marrying without the family's consent.
In November, a newly-wed couple in Sindh province who married without the consent of their elders were killed allegedly on the orders of a jirga (village elders).
More Pakistani women are murdered at the hands of close family members for supposed immoral behaviour than Pakistani civilians are killed by terrorism, the report said.
The Human Rights Commission of Pakistan (HRCP) has reported an average of 650 honour killings annually over the past decade. But since most go unreported, the real number is likely to be much higher, the report said.