Begum Bibi, who lived in Khyber Pakhtunkhwa's Shangla village, said she killed Gulbar Khan because "he didn't respect family and relations".
The woman claimed that Khan had been repeatedly assaulting their daughter-in-law for the past three months while their son was away, The Express Tribune reported.
The rape victim's husband, a Frontier Corps soldier, said he knew his wife's ordeal "but due to parental respect, I couldn't kill him, but informed my mother that I will leave home after my return from training," the report said.
"I decided to kill him when he refused to give up his evil practice," Begum said.
Police yesterday produced Begum, her daughter-in-law and son before a court which remanded them to judicial custody.