03/01/2013 10:19 EST | Updated 05/01/2013 05:12 EDT

Supreme Court of Canada upholds decision in bathtub murder conspiracy

OTTAWA - The Supreme Court of Canada has dismissed the appeal of a young man convicted of conspiring to help two teenaged girls drug and drown their alcoholic mother in a bathtub.

In doing so, the high court clarified the stage at which someone becomes part of a conspiracy.

A lower court convicted the man — whose name is protected under the young offender law — of conspiracy to commit murder for his role in the 2003 death of his girlfriend's mother

The court heard he suggested that the sisters — who were 15 and 16 years old at the time — ply their mother with alcohol and pills in the bathtub of their Mississauga, Ont., home.

Police uncovered a chat log between the young man and his girlfriend from a week before the murder.

"Your mom gets Tylenol 3's, right?" the man wrote.

"Probably," his girlfriend replied.

"Seriously, you should include them in the game plan," he wrote back.

"I'm not talking 20 here. I mean like five."

The dead mother was later found to have an amount of codeine in her blood consistent with the consumption of between four and six Tylenol 3 tablets.

The young man agreed to help the sisters come up with an alibi after the murder. He also suggested ways they could mislead police, suggesting they buy movie tickets for an hour before the murder took place to create a paper trail.

"I'm involved this much, I'm willing to help you out with any of it," he told his girlfriend in their chat.

He later told police his comments were not meant to be taken seriously.

The girls each received 10-year sentences, but have since been released.

The young man was found guilty of conspiracy to commit murder and received an 18-month sentence. Ontario's appeals court upheld the conspiracy conviction, but reduced the sentence to eight months confinement and four months under supervision.

The Supreme Court was essentially asked to determine the stage at which someone becomes part of a conspiracy — a legal question that had been answered differently from province to province.

Alberta and Quebec took a narrower approach, limiting it to people who aid or abet the agreement to commit a crime.

British Columbia and Ontario took a broader approach, expanding it to people who knowingly did something to make it easier for the conspirators to commit a crime.

The Supreme Court justices opted for the narrower approach.

And Justice Michael Moldaver, in writing for the court, said the young man could have been in far deeper trouble, since the help he gave the girls "could well have led to a charge of first degree murder against him."