Transcript for Missouri Mother, Daughter Killed Family Members with Anti-Freeze

Thanks for joining us. A chilling whodunit almost too sinister to comprehend. It started with a seemingly ordinary family suffering two devastating blows -- the father and adult son suddenly dead. Supposedly of natural causes. But it wasn't until their daughter ended up near death in the emergency room that the pieces started falling into place. ABC's Deborah Roberts has our story. I'm horrible. I'm a horrible mother. Reporter: With 24-year-old Sarah clinging to life in a Missouri hospital, instead of being by her side, her mom Diane is in interview room number 3 at the Springfield police station. My husband died last year of a heart attack. Reporter: Answering questions about the recent deaths of her husband mark and son Sean. Do you have a son as well? No. Not anymore. He died. Reporter: Both found dead within months of each other. Now with Sarah on the verge of death, investigators are wondering whether this church-going organist is the victim of unimaginable tragedy or a cold-blooded killer. I screwed up everybody. I screwed up my whole family. Reporter: It all began here in the heart of the bible belt where Diane and mark are raising four kids in a small house. Tight quarters for what looks like a tight family. Diane is a nurse and church organist at redeemer lutheran. She's the family breadwinner. Mark, lead singer and guitarist of a local blues band. Did you get any sense of what their relationship was like? As far as I knew, mark's wife was great. Reporter: Charles Alexander is mark's close buddy and the drummer in the band. He loved them, he loved his family. It was a great family. Reporter: It's a shock to friends and family when he dies suddenly in his sleep. I was devastated. I was devastated. Did you have any inkling that anything was wrong? No. Reporter: The medical examiner rules the death due to natural causes. Diane moves her fourchildren into a new house, then months later, that coroner is back at her house again. This time for son Sean. I said, we just saw the coroner's van was here. Are you okay? Is everything all right? And she said, oh, yeah. My son died. Reporter: Her neighbors say she's surprised by Diane's response when offering condolences. Was she distraught? No, she said it to me just like that. "My son died." Very matter of factually. I was shocked. Reporter: Now only the women are left. Diane, daughters Rachel, Sarah, and 12-year-old Briana. I think they closed the doors even tighter. Because you didn't see them outside. I don't know how they got their mail because we didn't see them at the mail box. Reporter: It's just the following June when tragedy seems to strike again. Diane rushes 24-year-old daughter Sarah to the E.R. Nip 11, where is your emergency? Reporter: But this time someone claiming to be close to the family calls in a tip to police, hinting that the pious church organist may harbor an unholy family secret. What did the caller say exactly? That Diane stouty might be responsible for two or three homicides. Brought up mark's death, Sean's death being very close proximity to each other. Also spoke about the potential that Sarah was going to die as well. Reporter: The detective assigned to investigate, Neil mcamus, visits the hospital where he gets troubling news from Sarah's doctor. He said that he was suspicious. That it was a possible poisoning case. Your radar goes up? Definitely. Reporter: Mcamus figures it's about time for Diane stouty to answer questions. She voluntarily comes to the station. I don't know what I can tell you. Reporter: With cameras rolling, he asks about her life and marriage. Quickly revealing a reservoir of resentment. We were still married, but -- it was not what you'd call a good marriage. I was unhappy. Reporter: He shifts from questioner to sympathizer. I'm a believer myself. So I understand where you're coming from on that. Reporter: And elicits a bombshell confession. To put it really short and sweet -- I knew they were drinking anti-freeze. Reporter: And the chilling details pour out. Diane stouty describing how she slipped anti-freeze into her family's drinks. How much would you put in? Couple of teaspoons, maybe. I couldn't believe what I was hearing. There were times I didn't know what to say or how to react. Reporter: Apparently she'd had it with her husband, saying she considered him an abusive deadbeat. By then I hated his guts. He would throw things at me. He would throw things at the kids. Reporter: Her children Sean and Sarah, both burdens in her eyes, not living up to mom's expectations. Both Sean and Sarah would just basically -- I don't know -- trash the house and never helped support or even contribute. Sean would be interfering with whatever I would do. So he was just a constant bother? Wouldn't leave you alone? He was more than a bother. A pest? Would that be a good word? It was more than that. I'm not a perpetual killer. I'm just stupid. I regret doing it. I really do. Reporter: But it's too late. Diane's arrested. Case closed? Not by a long shot. With Sarah still clinging to life across town, police are busy combing through the stouty home. The anti-freeze in plain sight. They also find an unassuming purple diary that belongs to Diane's other daughter Rachel. Appeared to be a journal entry written by Rachel knowing that mark and Sean were getting ready to be killed. Reporter: That journal entry dated June 13th, 2011, nearly a year before mark stouty died. It reads -- It's sad when I realize how my father will pass on in the next two months. Sean, my brother, will move on shortly after. It will be tough getting used to the changes but everything will work out. At first you're thinking, this is a mother, evil mother, who's tried to wipe out her family. And now you discover her daughter might be involved helping her? Yes. Reporter: Confronted with her diary, Rachel, always her mom's favorite, reveals the awful truth. That she was also mother's little helper. When did you guys come up with this plan? I mean, we talked about it like Christmas. Reporter: In another interrogation, Rachel tells police he and mom Diane weren't exactly finished. The next target, the youngest, 12-year-old Briana. When were you guys going to kill Briana? Some time after Sarah. Two people dead, two in jail, one in the hospital -- Reporter: The tale hits the news like an f-3 twister. A mother and daughter both arraigned for murder. The Springfield community is in shock. When I heard that he was poisoned? Then I just cried. I collapsed. I can't believe this woman did this. Reporter: But no one's more surprised than this young woman. I assumed it wasn't true, they were innocent people being blamed. Reporter: Sarah stouty sharing her story for the first time, defying all the odds, surviving a vicious poisoning and living to tell the tale. Do you still consider them family? Not anymore. I consider them as killers. Who hate me. Reporter: She had to relearn to walk and talk. I'm right here. Reporter: Her speech still stunted from the irreversible brain damage she suffered. They really planned this heinous crime. I was shocked. I just felt like I wanted to strangle my mom. Because of what she did. 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 and there was nothing at that point to indicate anything malicious. Neither one of them knew about the other deaths. Reporter: As for the medical examiner who critics say bungled the bodies of mark and Sean, missing obvious signs of foul play, they defend their work on the case. And yet for all the pain and loss, Sarah says she harbors no ill will against her mother, who in the end pleads guilty. Sarah was even brave enough to face her mom in court on the day she was sentenced to life in prison without parole. I forgive my mom. What changed? As time went by, I matured. Reporter: Her sister Rachel pled guilty to lesser charges and will be eligible for parole when she turns 65. Sarah smiles fondly when sharing photographs of a life that seems like a distant memory. That little boy here is my brother Sean. Reporter: Deborah Roberts for "Nightline" in Springfield, Missouri.

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