A Montreal couple and their son who were convicted of first-degree murder in the so-called honour killings of four female family members are appealing for a new trial.

Mohammad Shafia, his wife Tooba Yahya and their son Hamed, filed a 110-page factum with the Ontario Court of Appeal, arguing Justice Robert Maranger failed to intervene when the Crown presented arguments that they believe improperly swayed jurors in their decision-making.

The document also questions the testimony of University of Toronto Prof. Shahrzad Mojab, an "honour killing" expert who testified on behalf of the Crown.

The three accused were each given an automatic life sentence with no chance of parole for 25 years for their roles in the  deaths. 

The bodies of Zainab, 19, Sahar, 17, and Geeti Shafia, 13, along with Mohammad Shafia's first wife, Rona Mohammad Amir, 50, were found in the family's Nissan, submerged in a lock on the Rideau Canal on June 30, 2009. The prosecutors at their trial said the three accused felt the sisters and Amir had been acting dishonorably by not following family rules.

A hearing on the appeal is scheduled for Dec. 14.

Mojab 'overwhelmingly prejudicial'

In the factum, 13 pages are devoted to the testimony of Mojab, which lawyers say invited cultural stereotyping.

The Shafia family lawyers argue the Crown too heavily relied on Mojab's testimony and Maranger erred in allowing it to be submitted as expert evidence.

"Even assuming that the jury may have legitimately been assisted by evidence about Afghan cultural norms or concepts of honour, the way in which Dr. Mojab's evidence was presented exceeded that purpose and created enormous prejudice," documents filed with the court say.

Lawyers say trial judge erred

Lawyers for the Shafia family claim the judge made a number of mistakes throughout the trial.

Among their concerns was Maranger's instruction to jurors regarding police statements the Shafia family members gave following the discovery of the car in the Rideau Canal.

In his testimony, Hamed admitted to giving police false information, but both Mohammad and Tooba maintained the statements they gave to police were true.

The lawyers argue the judge erroneously instructed jurors to consider if they felt Mohammad and Tooba's statements were false as further evidence of their guilt.

None of the allegations made in the appeal documents have been tested in court.