Hannah Louise Glover, 30, initially pleaded not guilty to the murder of Tara Marie Morrison, 38, at a West Australian home in April 2024.
But on Thursday, Glover changed her plea to guilty a day after her ex-prison lover gave evidence about the killer's jail cell confession for the jury.
Glover and her then-boyfriend, James Anthony Sorgiovanni, 29, allegedly stabbed Ms Morrison in her head, neck, body and hands at their Busselton home, about 220km south of Perth.
Sorgiovanni, who has pleaded not guilty, remains on trial in the WA Supreme Court.
Police found Ms Morrison's body at Glover's home stuffed into her daughter's wardrobe.
She had about 40 wounds, along with grazes and bruising that could indicate she was restrained.
Blood was found splattered on the walls, doors and the floor.
The court has previously been told Glover, Sorgiovanni and Ms Morrison had been using illicit drugs before the killing and were involved in a love triangle.
Glover and Sorgiovanni had been in a relationship for a few months, and before that, Glover was in a sexual relationship with Ms Morrison.
Sorgiovanni was allegedly jealous of Ms Morrison.
Glover's prison lover, who cannot be named for legal reasons, said the mother confided details about the "brutal attack" while the pair were in the same prison in the months after the alleged murder.
"Hannah told me that Jimmy held Tara's hand behind her back and that's when Hannah grabbed her by the hair and stabbed her in the neck," she said.
The woman used her right hand and made a stabbing motion three times to replicate for the jury what Glover did when they were speaking in a cell.
Glover will be sentenced at a later date.
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