Mom, Daughter Were Going to Poison 12-Year-Old Girl Next: Part 5

Rachel Staudte told police that her 12-year-old sister was next because she wouldn't be able to take care of her.
5:12 | 01/29/17

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Transcript for Mom, Daughter Were Going to Poison 12-Year-Old Girl Next: Part 5
Reporter: You are looking at a woman who defied all the odds, and lived to tell. Surviving a vicious poisoning. This is where I sleep. Reporter: 24-year-old Sarah staudte has had to relearn to walk and talk. Right here. Reporter: Her speech still stunted from the irreversible brain damage she suffered. Tonight for the first time, she's speaking out about the horror and betrayal. Sarah lost her father mark suddenly in 2012 and brother Shaun just five months later. You found him dead. Yes. Reporter: That must have been horrifying. Traumatizing. Reporter: And more traumatizing, learning that her mom Diane was not her protector but a predator. Poisoning them all with antifreeze. And the icing on the cake, her younger sister Rachel was part of the plot. I was asking, why? Why did my mom and sister kill my dad and brother, and harmed me? Reporter: Did you believe it, or did you sort of assume, this can't be true. I assumed that it wasn't true. They were innocent people, being blamed. Reporter: But any trace of sympathy evaporates after Sarah reads in the newspaper why her mother poisoned three family members -- she simply despised them. They really planned this heinous crime. I was shocked. I just felt like I wanted to strangle my mom, because of what she did. Reporter: The big question, why did Diane bother to rush Sarah to the hospital after leaving her son and husband to die at home? In a follow-up interview with a clearly rattled Rachel at the green county jail, detective Mcamis wants to know. When Sarah got so bad, why did you take her to the hospital? Reporter: Was it some last minute tug of Christian conscience? Apparently, no. I didn't want another one to die in the house. And why is that? Because houses are nasty after somebody's died in it. I get a lot of nightmares. Reporter: But what Rachel tells police next may be the most heinous of all. She and mom Diane weren't exactly finished. Brianna, was she next? When were you guys going to kill Brianna? Some time after Sarah. Reporter: Brianna. The youngest member of the staudte clan. A tender 12 years old, too young to drive, but old enough to drink her mother's poison, antifreeze, in her root beer. They were going to kill the little girl as well? 12-year-old girl. Reporter: What's wrong with these people? What was the reason for Brianna? Because I know there's no way in hell I'd be able to take care of her. I can't take care of me, so how could I ever take care of her? She had described Brianna as a burden, they didn't want around the house. That was their explanation. Reporter: Four people, they would've killed four people in this house if they could've? Correct. Rachel said that her mother was the only one that understood her. They could relate to each other, and it was just going to be those two. Reporter: Do you think this was just a case of a mom and daughter who just in some sick way wanted to have a simpler life? I don't think there's going to be any acceptable remotely rational explanation. The question is, how could this have possibly happened? Reporter: Did the police blow this thing? Could they have prevented Sarah's poisoning by connecting these dots? I think it's easy to go back now. There was nothing, at that point, to indicate anything malicious. Neither one of them knew about the other deaths. Reporter: And what about the medical examiner's office, who critics say bungled the bodies of mark and Shaun by missing obvious signs of foul play? They defend their work on the case. When you have physical findings at the autopsy which match the story that was given, typically other organs aren't looked at microscopically. Reporter: Saying antifreeze is difficult to detect unless you're looking for it. Later tests did confirm that Shaun's body had the presence of the killer chemical ethylene glycol, found in antifreeze. That news came a bit late for Sarah. Still, she's learned to accept her new reality, living in an assisted care facility. More alone than ever. Do you still consider them family? Not anymore. I consider them as killers. Who hate me. Reporter: You're angry. I feel like I want to slap both of them, and calling them "B" words. Reporter: Their family plot uncovered, perhaps it was to be expected that the mother-daughter murderers would eventually turn on each other. And sure enough, Rachel made a last-ditch effort to save herself with a plea deal. Reaction to a guilty plea from Rachel staudte, the Springfield woman charged with poisoning three of her family members. Reporter: Throwing her mom under the bus. That's crucial. The fact that her daughter has pled guilty, seems ready to testify, could in and of itself be enough to convict her and potentially send her to death row.

This transcript has been automatically generated and may not be 100% accurate.

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