Francesca St-Pierre was beaten to death in August 2007. (CBC)

A Montreal teenager who two years ago brutally beat a fellow teen living in his group home has been given a six-year sentence that includes three years of court-supervised probation.

After the 17-year-old was sentenced Monday in Montreal youth court the victim's family said they plan to sue Quebec's youth protection services for failing to do its job.

Francesca St-Pierre, 14, was found dead near a bike path in Montreal's northeast sector in August 2007 after being reported missing a day before.

The accused, who cannot be named because of his age, was 15 at the time.

The youth court heard he followed St-Pierre from the group home and beat her to death with a baseball bat.

Quebec Superior Court Justice James Brunton said the accused is bipolar, suffers from Tourette's syndrome, attention-deficit disorder and a God-like superiority complex, and was medicated.

Brunton said the teen's medical conditions were key in deciding on the six-year sentence, which includes three years in closed custody and three years of probation.

The teen smiled as the judge pronounced the sentence.

Francesca's family was livid at the sentence and called the accused "garbage" once the judge left the courtroom.

The victim's half-sister Corinne Moreau said it made no difference whether the accused was medicated or not because she believes he knew what he was doing.

But Moreau said she understands he is a product of a dysfunctional youth protection service, where poor families trust officials will do the right thing.

"Take your pills and keep quiet. It's in that sense that I'm very angry with youth services. They say they protect children, but they don't do anything, and they're worse than the parents," she said.

Moreau has hired a lawyer to help her launch a lawsuit against youth services.

With files from The Canadian Press