The Crown said she was unhappy in her marriage and was motivated to murder by the prospect of cashing in the $250,000 life insurance policy.
It is alleged Milner plotted the best ways to kill her husband; buying drugs under false names, asking friends for views on poisoning methods, and even offering to pay one of her sons and others $20,000 for a hitman to kill Mr Nisbet.
Mr Kearns told the jury today how he had caught Milner crushing pills the night she was first accused of murdering his step-father.
"I called her a murderer," he said.
"I knew what she was doing. I basically said, 'You're sick, you're sick in the head. I don't want you as my mother any more'."
Mr Kearns - then aged 18 - told the court his mum had talked several times about killing Mr Nisbet, even discussing methods which included putting crushed glass in mashed potatoes and poisoning him with drugs and sleeping pills.
Initially, he "laughed it off - along with everyone else".
"I didn't think my own mother would be capable of that."
The night he stumbled across his mum allegedly crushing up a green/blue powder on the kitchen bench and putting it into clear capsules, he said she looked "shocked to see me ... a guilty look."
She then broke down in tears, allegedly saying, "I'm not going to do it, I'm not going to do it", he said.
That night, April 15, 2009 Mr Nisbet was admitted to hospital for a second time that day, feeling unwell from what he thought was an allergic reaction to a spider bite.
The Crown claims Milner had just tried to murder him - once in the morning, once at night.
Within days, Mr Kearns moved out of the home, telling Mr Nisbet on his way out that Milner was trying to kill him. He laughed it off, Mr Kearns said.
A month after he died, Mr Kearns went to police with his concerns.
"I couldn't live with it any more."
During cross examination, the court heard of an explosive text exchange between Mr Kearns and his mother after he had gone to police with his suspicions of foul play. The issue of the $20,000 to "get rid" of Mr Nisbet was raised.
Mr Kearns told the court he accepted he was trying to "bribe" his mum by raising the subject, but said he hadn't been serious given he had already gone to the police.
Pressed on it further by defence counsel Margaret Sewell, he became agitated: "What about what did she do to me? I didn't take any money away from her - she took everything away from me. Everything."
Ms Sewell put it to him that Mr Nisbet was scared and threatened by Mr Kearns, and didn't want him in the house.
Mr Kearns took exception to the line of questioning.
"She (Sewell) doesn't know me and she doesn't know Phil," he told Justice David Gendall.
"Phil and I got on well. That's that."
Friction between him and Mr Nisbet was caused by the "manipulative" actions of Milner, he said.
The trial continues tomorrow.
Text messages between Kearns and Milner:
MILNER: "I promise I did not do it. I could not live with myself."
KEARNS: "You tried it before."
AFTER MILNER FINDS A SUICIDE NOTE, HE TEXTS HER: ``I want to see the suicide note before I believe it was him.''
DURING ANOTHER ANGRY EXCHANGE, KEARNS RESPONDS: "Bring it on murderer."
KEARNS: "I'm going to make a [police] statement against you now."
MILNER: "Cool. Go make an idiot of yourself. Have you heard of blackmail?"
KEARNS: "Have you heard of murder?"
