The accused, Iftikhar Ahmed, hit his 21-year-old sister Aasia Bibi with bricks, killing her on the spot after he found her with a youth from the neighbourhood on the rooftop of their house in Kasur district, about 60 kms from Lahore, on Wednesday night, the girl's father said in the police complaint.
The girl's paramour had managed to escape from the spot. Disapproving his son's action, the father said, "I loved my daughter. She could have been admonished."
Raids are being conducted to arrest Ahmed, the police said. In May, a 25-year-old pregnant woman was brutally beaten and stoned to death outside the high court in Lahore by her father and brothers for marrying the man of her choice.
Condemning the incident, rights activists have appealed to the government to pass a legislation in parliament, barring the family members from becoming complainants in honour killing cases.
"The state should be a complainant in honour killing cases. The complainant from the victim's family often pardons the killer and the case is settled without any punishment to the accused," activist Abdullah Malik told PTI.
Last year, 860 women became victim of honour killing in different parts of Pakistan. In most cases the culprits escaped punishment after being pardoned by complainants who happened to be their close relatives.