A jury in the High Court at Rotorua yesterday found cousins Courtney Patricia Churchward, 18, and Lori-lea Waiora Te Wini, 15, guilty of the murder of John Rowe, 78, at his Opotiki home last year.
They beat him to death with his walking sticks.
Te Wini, who was 14 at the time of the murder, is believed to be the second youngest female killer in New Zealand's history after Renee Kara O'Brien, also 14, was convicted of murder for a hammer attack that killed Kenneth Pigott near New Plymouth, in March 2002.
Simon Gilmore, the partner of Mr Rowe's daughter Wendy, said the trial had highlighted the need for discipline and self-discipline within families and young people.
"We have just got to crack down on this youth crime."
Speaking to reporters after the verdicts, Ms Rowe agreed. "We have got to get people off the streets and learn some respect for people, especially older people who have given to society -- paid their rates."
Mr Rowe senior had come to New Zealand from Devon in the UK 40 or so years ago for a better life, she said.
"Look how it ended for him," Ms Rowe said.
Ms Rowe and her brother Patrick greeted the guilty verdicts with relief but said that was tempered by the knowledge their father's death should never have happened.
"He was living quietly, not annoying anyone, he was just a wonderful man living out his last years," Ms Rowe said.
The evidence heard during the trial was abhorrent, they said. "We were warned but we didn't know the horror of it would be to that extent," Ms Rowe said.
The worst part had been the graphic scientific and pathology evidence which had focused on the extent of the wounds their father suffered when he was battered with his walking sticks by teenagers.
The teens are to be sentenced by Justice Geoffrey Venning on December 18.
The officer in charge of the case, Detective Inspector Rob Jones, the manager of crime services for the Bay of Plenty, said Mr Rowe's killing had been a horrible tragedy, not only for his family, but the families to the two girls and the girls themselves.
"But it has shown we have a justice system and that it is in fact fair and on this occasion delivered a guilty verdict to both the accused."
