In an extended interview with Associated Press (AP), Rajhu said he loved his sister, and had made her promise not to marry the Christian man she had been seen with.
"I told her I would have no face to show at the mill [his workplace], to show to my neighbours, so don't do it. Don't do it," he said.
"But she wouldn't listen."
Tasleem married the Christian, a man named Jehangir, with another brother as a witness.
Jehangir converted to Islam to appease her family, but it wasn't enough.
"I could not let it go. It was all I could think about. I had to kill her," Rajhu said. "There was no choice."
A week later, on August 14, Rajhu shot Tasleem while she was sitting in their kitchen with her mother and sister.
"There was no yelling, no shouting," Rajhu recalled. "I just shot her dead."
Various interviews conducted by AP with members of the Muslim community in Lahore revealed that there is widespread support for Rajhu's actions.
"I am proud of this man that he has done the right thing, to kill her," one neighbour said. "We cannot allow anyone to marry outside our religion. He did the right thing."
Evern Rajhu and Tasleem's father blames his daughter, and is afraid of what their wider family will think of her marriage.
"My family is destroyed," he said. "Everything is destroyed only because of this shameful girl. Even after death I am destroyed because of her."
Meanwhile the Christian community in Lahore say they are living in fear since Tasleem's death. Gunmen have fired bullets into the homes of Christians living in Jehangir's area – he fled the night Tasleem was killed.
"We have been scared since the killing took place," one Christian neighbour told AP. "There are just a few houses of Christians here, but we have nowhere else to go."
Christians are protected under Pakistan's constitution, but many face severe persecution for their faith. The 'Forced Marriages and Inheritance Deprivation' report from the Karachi-based Aurat Foundation claims that up to 700 Christian girls are married forcibly each year and forced to convert to Islam.
So-called 'honour' killings are also on the rise. According to the Human Rights Commission of Pakistan, there were 1,184 victims in 2015, all but 88 of whom were women.
A report released in May by the United States Commission for International Religious Freedom (USCIRF) found that the Pakistani government last year "continued to perpetrate and tolerate systematic, ongoing, and egregious religious freedom violations".
USCIRF has recommended Pakistan be designated a 'country of particular concern' by the US since 2002 and its report was damning about Pakistan's failures. "For years, the Pakistani government has failed to protect citizens, minority and majority alike, from sectarian and religiously-motivated violence," it said.
"Pakistani authorities also have failed to consistently bring perpetrators to justice or take action against societal actors who incite violence."
Lahore is considered a particularly dangerous place for Christians to live. Nasir Saeed, director of CLAAS-UK, a charity dedicated to helping persecuted Christians in Pakistan, has branded Lahore "the province where Christians suffer most".
On Easter Sunday this year, a suicide bombing in a Lahore park killed 72 people. It was the deadliest attack in Pakistan since the massacre of 134 children at a military school in Peshawar in December 2014.
Though many more Muslims than Christians were killed in the attack, the faction of the Taliban which claimed responsibility for the blast confirmed that Christians were the target.