The mother of a 14-year-old girl murdered more than seven years ago outside Hope, B.C., is stunned one of the men convicted of her slaying will receive a new trial.

Dustin Moir was found guilty in February 2010 for the first-degree murder of Chelsey Acorn. But the B.C. Court of Appeal has ruled the trial judge made a mistake.

"I'm speechless. I can't believe we have to go through this again," Lisa Acorn told CBC News after hearing of the appeal court ruling.

Acorn thought her family could finally move on after last week when Moir’s father, Jesse West, 60, was also found guilty of first-degree murder in Chelsey’s death. But Moir himself will now get another chance to prove his innocence.

"I definitely didn't expect it, that's for sure. We thought we were able to start to move forward and put some closure to this. I guess not yet. We still have another journey to go on."

In 2006, Chelsey Acorn's body was found along the Coquihalla River, north of Hope.

Moir told an RCMP officer posing in a Mr. Big sting that he had helped kill the teen, but later he testified he only helped his father bury the girl's body.

Moir claimed he felt compelled to exaggerate his involvement in order to stay in Mr. Big's criminal organization.

The B.C. Court of Appeal ruled there were two versions of events, and the trial judge did not instruct the jury on the proper use of inconsistent statements from witnesses.

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Chelsey Acorn's mother, Lisa Acorn, hugs a family member outside court after the conviction of Jesse West Jan. 21. (Emily Elias/CBC)

Lisa Acorn is not looking forward to a new trial.

"Well, it's hard, but you gotta do it. It's for Chelsey, it's not about us. Got to be there for her."

No trial date has been set.

With files from the CBC's Robert Zimmerman