Zeenat Rafiq, who was 18, had been burnt and there were signs of torture and strangulation, police told BBC Urdu. A post mortem examination may establish if she was still alive when she was set on fire.
Police Superintendent Ibadat Nisar said officers were looking for her brother who is "on the run". Her mother was found in the house with the body.
"Her mother has confessed to the crime but we find it hard to believe that a 50-year-old woman committed this act all by herself with no help from the family members," he said.
Neighbours contacted authorities after hearing screaming, but Ms Rafiq was already dead by the time police arrived, BBC reporter Saba Eitizaz says.
Ms Rafiq and her husband, Hassan Khan, married a week ago through the courts after eloping. They went to live with his family.
"When she told her parents about us, they beat her so severely she was bleeding from her mouth and nose," Mr Khan told BBC Urdu.
"Her family lured her back, promising reconciliation and a proper wedding reception. She was afraid, she said 'they are not going to spare me'. She didn't want to go but my family convinced her. How were we to know they would kill her like this?"
Nearly 1,100 women were killed by relatives in Pakistan last year in so-called honour-killings, the independent Human Rights Commission of Pakistan (HRCP) says. Many more cases go unreported.
Violence against women by those outside the family is also common.
Najam U Din, a joint director of the HRCP, said that societal attitudes had not changed in line with greater education and freedom for young women.
"So when women become more assertive, more reluctant to be content with submissive survival within the family - for example when they insist on studying further, or when they want to take independent decisions about themselves - then the society does not allow it."
Protests over new law
Punjab province, where the two latest attacks happened, passed a landmark law in February criminalising all forms of violence against women.
However, more than 30 religious groups, including all the mainstream Islamic political parties, threatened to launch protests if the law was not repealed.
The Council of Islamic Ideology, which advises the government, then proposed making it legal for husbands to "lightly beat" their wives. It was criticisedas a result.
Religious groups have equated women's rights campaigns with promotion of obscenity. They say the new Punjab law will increase the divorce rate and destroy the country's traditional family system.