GREELEY, Colo. -- A Johnstown teenager who admitted to helping her boyfriend kill her 7-year-old half-sister during a re-enactment of "Mortal Kombat" was sentenced Wednesday to 18 years in prison.

But her prison sentence will be suspended if she successfully completes a six-year sentence in the Youthful Offender System, the judge ruled.

Heather Maria Trujillo, 17, was charged as an adult, along with her boyfriend, Lamar Roberts, 17, in the Dec. 6. death.

Trujillo and Roberts were baby-sitting Trujillo's half-sister, Zoe Garcia, when Zoe suffered fatal injuries, police said.

Trujillo and Roberts were charged with felony child abuse resulting in death. Trujillo pleaded guilty in May to a lesser charge of child abuse, negligently causing death.

As part of the plea agreement, Trujillo promised to testify truthfully against Roberts.

While handing down his sentence, Weld County District Court Judge Marcello Kopcow said he does not believe the girl's death was an accident. He said he was not surprised that Trujillo had put herself in this position.

"Her lifestyle was spiralling downhill," Kopcow said.

He said in the youthful offender system, with a controlled regimented environment, Trujillo will learn skills, self-discipline, self-esteem, and problem solving.

"And I believe that all of those things are something that you have lacked in your 17 years on this earth," Kopcow told the teen. "I hope that after six years in the youthful offender system you will be a young lady who can give to this community rather than take from this community."

Heather and Zoe's mother, Dana Trujillo, spoke in court on behalf of her daughter.

She said Heather has never been a problem and losing her behind bars for 18 years would be devastating, since she's already lost her younger daughter.

"Heather's a huge part of my life. And this is a great loss," Dana Trujillo said between sobs.

"I think she's realized family's not (something) to take for granted. That they are your family and that you are to love and cherish them everyday," Heather's father, James Henry, said.

He called the deferred sentence "very fair."

Defense attorneys argued that the adults in Heather's life never gave her structure or discipline.

The Department of Social Services in six different counties -- Adams, Arapahoe, Denver, Jefferson, Lincoln and Weld -- investigated the family but never took the kids away, said defense attorney Jerry Manzer.

He said there was no parental supervision in the home, which was rife with drug and alcohol problems.

"The adults in their lives let them live as husband and wife. They shared the same bed," Manzer said, speaking of Heather and Roberts.

Heather Trujillo told Johnstown police that she and Roberts were imitating the Mortal Kombat video game when they began attacking Zoe. An arrest affidavit said the two wrestled, kicked, punched, karate-chopped and body-slammed Zoe against the floor.

The girl asked them to stop, but Roberts told police he did not stop because he was "drunk," according to the arrest affidavit.

When Zoe lost consciousness, Roberts and Trujillo tried to wake her up by placing her in a bathtub, the affidavit said. They also cracked open an egg and put it in her mouth to make sure Zoe was not "messing with them."

Trujillo and Roberts tried reviving her under running water before calling the girls' mother and 911, according to court affidavits.

The teens waited for 15 minutes before contacting police because they were afraid "the cops were going to get mad" because teens were watching a child, the affidavit said.

Autopsy records showed Zoe suffered a broken right wrist, bleeding in her brain, and more than 20 bruises and injuries to her neck and spine. The Weld County Coroner's Office said she died from blunt-force trauma, specifically the bleeding and swelling of the brain.

Prosecutor Robb Miller said Trujillo had an active role in her sister's death "in either not calling 911 or not acting as an older sister should."

Roberts will appear in court for a preliminary hearing on July 10.