The Ontario Court of Appeal has dismissed a request for a new trial for a Montreal couple and their son who were convicted of first-degree murder in the so-called honour killings of four female family members in 2009.

"Charitably put, the evidence of guilt was overwhelming," Justice David Watt wrote in the decision released Wednesday, and co-signed by two other Court of Appeal justices.

Mohammad Shafia, his wife Tooba Yahya and their son Hamed launched an appeal earlier this year. After their convictions in January 2012, each had received automatic life sentence with no chance of parole for 25 years.

In court last March, the family's lawyers argued that the trial judge should not have allowed the testimony of University of Toronto Prof. Shahrzad Mojab.

Mojab's suggestion that honour killings are planned by fathers and oldest sons, often with the help of mothers, was prejudicial and "flat out prohibited," lawyer Frank Addario said at the time.

Lawyers also argued that Hamed Shafia should have been tried as a youth.

They said they had uncovered new evidence showing their client, who immigrated to Canada from Afghanistan with his family in 2007, was actually 17, a year younger than originally believed, at the time of his arrest in 2009.

The Ontario Court of Appeal heard the case because the original trial was held in Kingston, Ont., where the women were killed.

The bodies of Shafia and Yahya's three teenage daughters and Shafia's first wife were found in a car at the bottom of the Rideau Canal in Kingston, Ont.