The murder occurred in Taxila, 25 kilometres west of Islamabad on Tuesday, according to officials.
Underrepresented: Male, female voter disparity rising at alarming rate
Danish Ali, 20, was furious because his sister Asif Noreen, a 32-year old schoolteacher, had defied his instructions and cast her vote, local police official Naeem Abbas told AFP.
"According to a report filed by the deceased's father, Ali shot Noreen with his pistol and she died on the spot at their home," Abbas said.
Police were still hunting for Ali, who is on the run.
Ensuring political participation of women
Women's turnout is usually weak in the most conservative rural parts of Pakistan because of male perceptions that their place is in the home.
But incidents of women being killed for voting are rare, although hundreds of Pakistani women are murdered each year in cases of domestic violence or on the grounds of defending family "honour".
First party-based LG polls: Registered voters down by nearly 4,000
The Aurat Foundation, a campaign group that works to improve the lives of women in Pakistan's conservative and patriarchal society, says more than 3,000 women have been killed in such attacks since 2008.
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