Phoenix Sinclair's mother and her common-law partner have been found guilty of first-degree murder. (CBC)

A Manitoba couple who buried the body of the woman's five-year-old daughter in a shallow grave near the Fisher River Indian Reserve were sentenced to life in prison for the first-degree murder of Phoenix Sinclair.

The woman and her common-law husband will not be eligible for parole for 25 years, Court of Queen's Bench Justice Karen Simonsen ruled Friday afternoon.

Phoenix died in June 2005 on the cold basement floor of her home. Her body was found in March 2006, wrapped in plastic in a shallow, unmarked grave near the garbage dump on the Fisher River Indian Reserve.

Phoenix's mother, Samantha Kematch, and stepfather, Karl Wesley McKay, were convicted earlier Friday by the 10-woman, two-man jury, which had been deliberating since Tuesday in Winnipeg.

'It shouldn't happen to any little child.'—Hilda McKay, sister of stepfather convicted in Phoenix Sinclair's death

Kematch and McKay appeared to show little emotion when the verdict was read, but several people sitting in the public gallery let out a sigh while others were in tears.

McKay's sister, Hilda, said the couple got what they deserved, but she didn't feel justice could ever be served for what the little girl endured in her short life.

"We were all shocked to find out what happened to Phoenix," Hilda McKay said outside the courthouse. "It shouldn't have happened to her. Everybody knows that. It shouldn't happen to any little child."

Kematch and McKay had pleaded not guilty.

Defence argued it was manslaughter

During the trial, defence lawyers did not deny the abuse but argued there was not enough evidence to convict the couple of first-degree murder.

The competing defence teams had suggested that manslaughter may be a more appropriate sentence because it was not clear who had delivered the final fatal blow to Phoenix.

On Thursday, the jurors queried the judge on whether the Crown had to prove a defendant has a murderous state of mind while both performing an unlawful act and while failing to perform an action to prevent a crime.

Simonsen told the jurors they can establish state of mind through either avenue — not both.

The girl was a ward of Child and Family Services for much of her life until Kematch regained custody about a year before her death in June 2005.

A child welfare worker checked on the family in early 2005 but never saw Phoenix during the visit. No one ever followed up on the case.

During the trial, the court heard Kematch and McKay routinely beat Sinclair with their fists, feet and metal bars and forced to eat her own vomit. Phoenix was also choked until she passed out and was shot with a pellet gun. She was confined to a cold basement.

The court was told the girl had broken bones throughout her body when she died.

Teen son tells police about child's abuse

In a videotaped interview with police played in court during the trial, McKay said he found Phoenix's body on the floor of the basement on the day he had told her to "shut up" and threw her on the floor.

He said he had tried unsuccessfully to resuscitate her before wrapping the body in plastic and driving to the garbage dump where he buried the child.

Police did not look for Phoenix's body until one of McKay's teenage sons told authorities about the abuse he had witnessed.

After the couple were charged, McKay later led police to the grave where her body was found.

The couple are also accused of trying to pass off another child as Sinclair to convince welfare investigators and the RCMP that their daughter was still with the family — partly in order to apply for benefits in the girl's name.

Phoenix's death will be the subject of a public inquiry that is expected to begin in the new year and will examine Manitoba's child welfare system.

With files from the Canadian Press