A nurse said she thought "not again" when a baby suddenly collapsed the day after her twin brother had died at a neonatal unit, jurors have heard.
Lucy Letby, 32, is on trial accused of murdering seven babies and attempting to murder 10 others at Countess of Chester Hospital in 2015 and 2016.
Her nursing colleague and "mentor" told Manchester Crown Court how Child B had become "ill very quickly" and her sudden deterioration was "unusual".
Ms Letby denies all 22 charges.
It is alleged nurse Ms Letby injected air into the bloodstream of Child A, shortly after she came on shift on 8 June 2015, just over 24 hours after his premature birth.
The Crown alleges she used the same method to attack his sister, Child B, on the following night shift.
Paediatric consultant Dr Ravi Jayaram told the trial he "couldn't explain" the sequence of events that led to Child A's death.
He said it was very sudden and did not "fit with any disease process that I have seen, learned or read about", adding: "It didn't make sense to me."
He and senior colleagues "began to raise concerns" to hospital managers about Ms Letby regularly being present when the children in this case fell ill, the court heard.
He told the jury: "At that time we were being told that we shouldn't be saying such things and not to make a fuss.
"I didn't really have any hard evidence, but I wish I'd been more courageous at the time."
The nurse worked on the neonatal ward at Countess of Chester Hospital
Ms Letby's nursing colleague, who cannot be identified for legal reasons, recalled she was preparing medicines when the monitor alarm sounded at Child B's incubator.
The nurse said Ms Letby was the first to go to the cot and she called her for help.
Asked about Child B's appearance, she said: "She looked very ill. She looked very like her brother did the night before.
"Pale, white, with this purple blotchy discolouration. It was all over her body.
"I just remember thinking 'not again' - to see his sister with the same appearance."
A breathing tube was inserted and Child B "started to stabilise quite quickly", said the witness, who was giving evidence screened from the public gallery and the defendant.
She said: "Child A's deterioration was very sudden and to an unusual degree.
"Babies can be very poorly quickly but there is usually some indication that is happening. We had no undue concerns.
"To go from that is very unusual and then Child B had been good throughout the evening for me... then she became ill very quickly.
"She deteriorated very quickly and then this discolouration.
"You never want any baby to die. You want to help them go home to their families. That's always been my goal."
Child B recovered and was eventually discharged a month later, the court has heard.
Lucy Letby has been described as a "poisoner at work" on the ward
The witness could not explain why, when interviewed by police in 2018, she had not mentioned an unusual discolouration in Child A's appearance, but when later questioned about Child B she said her discoloured skin was similar to her brother.
She told Ben Myers KC, defending, that people on the unit were talking at the time about rashes but she was not influenced by anything anybody else said.
The nurse told the jury she could not remember who administered intravenous fluids to Child A shortly before his collapse but accepted she told police that another nursing colleague had "pressed start" in the process and Ms Letby had assisted with checks.
She said she acted as "mentor" to Ms Letby, who first came to the unit as a trainee around 2010/11 while studying at the University of Chester.
They became "good friends", she said, as Ms Letby went on to join the unit after she qualified.
Mr Myers said: "We know the allegations but your experience when working with her was she was highly professional?"
"Yes," the witness replied.
Mr Myers said: "And dedicated to the work she was doing?"
"Yes," agreed the witness.
The trial continues.