The court has upheld three first-degree murder convictions against Cathie Gauthier, who is serving life in prison with no chance of parole for 25 years.
Her trial heard that Gauthier and her husband Marc Laliberte were in financial trouble and had formed a murder-suicide pact.
Her husband and children were found dead after she made an emergency call on New Year's Day 2009.
Her lawyer argued that the judge gave the jury improper instructions on different points of law before they retired to consider their verdict.
The high court ruled Friday by a 6-1 margin that the trial judge acted properly in deciding what the jury was allowed to hear.
At her trial, the Crown argued that Gauthier planned the murder-suicide pact with her husband and supplied the murder weapon — poisoned drinks served by her husband that contained Gravol and oxazepam.
Gauthier contended that she did not buy the medication and that was in a dissociative state the night before.
But Gauthier wanted to put forth an alternative defence — that she abandoned the plan she had made with her husband.
The trial judge ruled that the alternative defence of "abandonment" should not be put before the jury, a ruling that was upheld by the Quebec Court of Appeal.
The Supreme Court concurred Friday.
"There is no cardinal rule against putting to a jury an alternative defence that is at first glance incompatible with the primary defence," Justice Richard Wagner wrote for the majority.
But in Gauthier's case, Wagner wrote that there was insufficient evidence that she communicated her intent to back out of the plan in a timely and unequivocal manner.
"The defence of abandonment therefore did not meet the air of reality test, and the trial judge was not required to put the defence to the jury."
Late in the morning of Jan. 1, 2009, Gauthier made an emotional 911 call from her home in Saguenay.
Between sobs, she requested an ambulance. The operator asked her what the problem was and she replied that her wrist had been cut.
When Gauthier was asked whether she had attempted suicide, she replied: "It was a pact; my husband killed our three children."
She then added: "We said that we wouldn't start 2009, but ..."
Gauthier told the operator she woke in bed lying next to her three children. Her husband was lying on the bedroom floor with his wrist slit.
She tried to revive her children but they did not respond.
Gauthier said she dragged herself to her kitchen to call for help, explaining that her husband had cut her wrist and drugged the family with sleeping pills.
"She supplied her spouse with the intoxicants he used to cause the children's deaths," Friday's ruling said. "She therefore had to do more either to neutralize the effects of her participation or to prevent the commission of the offence.
"For example, she could have hidden or destroyed the medication she had purchased, remained watchful and taken the children to a safe place for the evening, insisted that her spouse give her verbal confirmation of what he intended to do, or simply called the authorities."