A 13-year-old Alberta girl accused of slaying her family admitted to stabbing her little brother once, but said her much older boyfriend was the one who slit the child's throat and killed her parents.

The Medicine Hat girl, who spoke in a whisper that could barely be heard in the packed courtroom, took the stand in her own defence Tuesday at her first-degree murder trial.

She cannot be named under the Youth Criminal Justice Act.

She testified that the night in April 2006 when her family died, she had called her boyfriend Jeremy Steinke, then 23, and told him she wanted to kill her parents because they were mean to her.

She told the court in Medicine Hat that she had been only joking, but that night she woke up to the sound of Steinke breaking into her house. Her mother went downstairs and screamed, drawing her father and her eight-year-old brother out of their bedrooms, she said.

The girl said she could hear her father and Steinke fighting downstairs, where she later saw her mother lying in a pool of blood.

She said she raced back upstairs and grabbed a knife she had hidden in her closet in case she needed to protect herself. Steinke came up the stairs, staggering and covered in blood, and screamed at her to kill her brother, she said.

"He yells at me 'Stab him, just stab him! Slit his throat!'" she said. "I said, 'I can't, I can't,' and he said, 'You have to. I did this for you.'"

Weeping as she spoke, the accused said the little boy pleaded with her, saying: "I'm scared. I'm too young to die."

The girl testified she stabbed the boy once somewhere in his upper body, but couldn't do anything more. Steinke then grabbed the kitchen knife and slit her brother's throat.

She said she couldn't look, but heard the boy struggling to breathe.

"He was gurgling," she told defence lawyer Tim Foster.

During her testimony, the girl wore a blue blouse and black pants, and had her hair tightly pulled back behind her head. The conservative outfit is a stark contrast to the internet photos the girl posted online before the killings, which showed her dressed in tight black clothing, sucking on her finger suggestively.

The girl was 12 years old when the bodies of her parents and younger brother were found in their Medicine Hat home.

'Hypothetical' conversations about killing family

Earlier in her testimony, she said she and Steinke had "hypothetical" conversations about killing her parents.

She said they used to talk late at night about how much she hated her parents, who didn't approve of their relationship.

She said they discussed possible ways of killing them, including making it look like a murder-suicide or hitting them on the head while they slept and then burning their house down. She said the plans included killing her brother because he was too sensitive to be an orphan.

But the girl said she never took the conversations seriously and considered them no big deal because all of her friends talked like that. She called it "stupid talk."

"This wasn't unusual for my group of friends — talking about killing people."

The accused said that a week before the killings she snuck out of the house and went with Steinke back to his trailer where they had sex. She said they had sex again in a friend's apartment after the killings, only hours before her family's bodies were discovered by her brother's friend.

"I loved him so much. I thought it would bring us closer together," she said, when asked why she would have sex when she was just 12 years old.

The girl will be cross-examined by Crown attorney Stephanie Cleary on Wednesday.

Steinke also faces three charges of first-degree murder, but his trial date has not been set.

With files from the Canadian Press