Away from his mother, Nathan blossomed and became an honors student. Brae, who remained with her mother and stepfather, continued to thrive in school, becoming interested in student government, despite obstacles at home.
According to her lawyer, Troy Britt, Brae's dream was to go to law school someday, following in her stepfather's footsteps.
"That's what her dad, Timothy MacNeil, did. He went to law school. He was an attorney. And someday, she wanted to be a judge," Britt told ABC News.
But as their mother's condition worsened, she continued to take it out on her daughter, straining their relationship and pushing Brae to the brink.
"It got to the point where I had to move to Tucson, because I thought she was going to kill me," said Brae Hansen.
To escape her mother, Brae moved in with her brother, now a college student in Arizona. Six months later, a terrible tragedy took place.
"My mom called and threatened suicide. ... She said that she was gonna die. She was like, 'I need to talk to Brae,' " Nathan recalled of the night in June 2006.
It wasn't the first time his mother had threatened suicide, and Nathan said he perceived it as another empty threat.
"I didn't know that she had already taken a whole handful of pills," Nathan said.
The next morning, MacNeil found his wife dead in the office of their home.
"I guess she just wanted to say goodbye" said Hansen.
For two kids who sought their mother's love, her death was a devastating blow.
"I was bawling. I couldn't stop crying for days," Nathan said.
Despite their tumultuous and tragic relationship with their mother, the siblings seemed to be coping well, and a feeling of normalcy was restored. Nathan continued school at the University of Arizona, where he started his own computer company, and Brae moved back home with her stepfather, where she became used to having MacNeil all to herself.
But when her stepfather began dating and fell in love with another woman named Kim, Brae questioned the role of a new woman in her stepfather's life.
"He was basically going to cut me out of his life completely," she said in the videotaped police statement. "I knew he'd already chosen his girlfriend over me, and it hurt really, really bad because this was the man I thought loved me, and was my dad."
In July 2007, MacNeil was killed by four gunshots, and both stepchildren became suspects his murder.
But family, friends and investigators were all asking the same questions: What possibly could have caused the siblings to kill their seemingly beloved stepfather, who always stood up for them?