Erin Patterson, 50, is on trial in regional Victoria accused of three murders and one attempted murder for serving a death-cap mushroom laced meal to her estranged husband's family in July 2023.
She has denied all offences and claims the fatal poisoning of the Pattersons, her former in-laws, and Gail's sister Heather, 66, was "a terrible accident".
The couple had four children - Erin's estranged husband Simon, Matthew, Nathan and Anna Terrington - several of whom will give evidence to the jury throughout the five-to-six-week trial.
Ms Terrington was called on Wednesday morning and began sobbing minutes after she took the witness stand at the court in Morwell, southeast Victoria.
She spoke to her mother on the phone about 5pm on July 29, 2023, hours after she had been for lunch at Patterson's home.
"Mum said it went well," Ms Terrington told the jury.
"She said they had beef Wellington and that it was too much for mum and so dad finished hers."
About 11am the following morning, Simon called Ms Terrington and told her Don and Gail had been taken to hospital and had been experiencing vomiting and diarrhoea since midnight.
Simon said Erin had experienced diarrhoea "but was soldiering on at home", and that his parents were "worn out and tired", she said.
Later that day, Simon told Ms Terrington their parents were being transferred to Dandenong Hospital in Melbourne and they had been separated.
Ms Terrington said she travelled to Dandenong, in Melbourne's southeast, to be with her mother, arriving about 10.53pm on July 30.
She began to cry again as she described helping her mother Gail in hospital.
"I took her to the bathroom many times," she said, between tears.
Under cross-examination by defence barrister Sophie Stafford, Ms Terrington confirmed she and Patterson had fallen pregnant at the same time.
They had delivered babies three days apart, known as "the twins" in the family, the jury was told.
She also confirmed Patterson had delivered a bible reading at Ms Terrington's wedding 18 years ago, and that Erin and Simon had loaned her hundreds of thousands of dollars to help with her home payments.
Her brother Matthew Patterson said he went to Dandenong hospital on July 31 "tag team with Anna" and see his father and mother.
When he arrived Don had been moved to the intensive care unit, but was conscious and talking to two medical staff.
He said the staff had "Monash toxicology" written on their shirts and they were asking Don about what type of meal he had eaten.
Matthew then decided to call Erin Patterson, who had cooked the meal.
"I just wanted to see how she was doing, it was a polite simple answer from recollection," he said.
"I then asked her where the mushrooms from the dish had been sourced from, she mentioned that there were fresh mushrooms from Woolies and that there were dried mushrooms from a Chinese grocer or supermarket."
Heather and Ian Wilkinson's daughter Ruth Dubois told the jury she was confused to hear her parents had been invited for lunch at Erin's home.
"It was not something I would have imagined," she said.
"In response to my reaction, she said 'yes we were surprised also', that that had never happened before."
Mr Wilkinson, 71, made a full recovery and was discharged on September 21, but Heather and Gail died in hospital on August 4, and Don died on August 5.
The trial before Justice Christopher Beale continues.
