TORONTO -- A lawyer for a young woman who waged a campaign of taunts and sexual blackmail to pressure her boyfriend into killing a 14-year-old girl says a new trial is required to properly decide her case.

Melissa Todorovic was 15 when her boyfriend, David Bagshaw, stabbed Stefanie Rengel six times and left her to die in a snowbank outside her house on New Year's Day, 2008.

Todorovic never met Rengel, but the younger girl became the target of intense teenage jealousy since she had briefly dated Bagshaw two years earlier and Todorovic believed Rengel was spreading rumours about her.

Her lawyer, Brian Snell, is at the Ontario Appeal Court today asking for her first-degree murder conviction to be overturned or her adult sentence changed to a youth one.

Snell says police didn't conduct their interviews with Todorovic properly in accordance with her rights under the Youth Criminal Justice Act. He says the incriminating statements Todorovic made were relied on by the Crown at her trial so a new trial is necessary without the use of those statements.

Both the families of Rengel and Todorovic are at the Appeal Court today to hear the arguments.

Todorovic was sentenced as an adult to life in prison with no chance of parole for seven years - the maximum adult sentence for someone her age.

A youth sentence would see her serve a total of 10 years - six behind bars and four of community supervision.

Police brought Todorovic in for an interview in the aftermath of Rengel's murder and didn't properly explain her rights, Snell argues, including telling her that she didn't have to give them a statement and that if she did, she could have a lawyer there.

Todorovic told police it was more her idea than Bagshaw's to kill Rengel, that she knew he was heading to Rengel's house that night to kill her, that she called him 15 minutes after the murder asking, "Is she dead?", and then asked Bagshaw to come over to her house so he could demonstrate how he had killed Rengel.

She also told police she didn't think Bagshaw would take her seriously when she told him to kill Rengel, but after all Todorovic's comments incriminating herself, police placed her under arrest.

But Snell argues that Todorovic was already detained when she told police about her involvement and police already had grounds to believe she had committed a crime, which would trigger Todorovic's YCJA rights.

"Detectives testified emphatically that when they were taking the first statement they had no idea that the appellant might implicate herself in the homicide," Snell said in his written arguments.

"(But) it strains belief that two experienced homicide detectives would have approached the case with such naivety."

The detectives knew, Snell suggests, that if they said otherwise Todorovic's admissions wouldn't be allowed at trial.

Before they interviewed her, police had already learned Bagshaw said he didn't want to kill Rengel but was being pressured by Todorovic and they had already learned of the phone call in which Todorovic asked if Bagshaw had gone through with it, Snell writes.

"That there was a significant consequence to not having a lawyer present during these interrogations is evident," Snell writes.

"No lawyer would sit idly by when questions were asked which directly invited the appellant to incriminate herself."

The Crown is arguing that the trial judge was right to allow Todorovic's statements as evidence, as police only regarded her as an important witness before they started the interview.

The reams of messages alone, in which Todorovic threatens to break up with Bagshaw or withhold sex until he kills Rengel, were powerful evidence of Todorovic's guilt, the Crown argues.

"If the trial judge erred by admitting either or both of the appellant's statements, then such error occasioned no substantial wrong or miscarriage of justice," the Crown writes in its arguments.

"There is no reasonable possibility that the verdict would have been different."

Bagshaw, who was days away from his 18th birthday when he stabbed Rengel, pleaded guilty to first-degree murder.

His bid to have the Appeal Court sentence him as a youth instead of an adult was dismissed three years ago.