"The moment she allowed another man to raise his hand to you ... she stopped being my sister," she read.
"She stopped being your mother and it was the moment I decided she was dead to me."
Sitting in the dock with her husband, the mother wiped away tears.
An uncle told the court he was "haunted not just by the brutality of Joseph's death but by their sheer lack of compassion".
"The brutally cruel death of Joseph has pushed my family's emotions to new uncharted levels of grief and despair which continues to consume us in mind, body and spirit," he said.
During the trial, Joseph's adult half-brother testified he saw the mother aggressively shake the boy so his head was "flopping back and forth".
He told the court the stepfather also shut the boy, naked and screaming, into an Esky full of ice and held the lid down for more than five minutes.
"I've never heard a person scream like that.
"I went outside because I couldn't take it any more," he said.
On other occasions, a tennis-size ball was taped into the boy's mouth, his head was slammed into a wardrobe door and his penis flicked with a wooden spoon, the court heard.
Joseph died three days after paramedics were called to the couple's home to find him in cardiac arrest, with grazes on his cheeks and temple and a bruise on his forehead.
The couple claimed Joseph tripped over a rope tied to the family's dogs.
Ms Cunneen said that was an "absurd explanation" and the couple should receive life sentences.
The mother's lawyer Eric Wilson SC said even though she previously had thoughts about killing her son, she had "no intention to kill" him when he died and it was "a moment of exasperated lack of control, loss of temper".
The stepfather's lawyer Alissa Moen said he would have been unaware of the abuse because he took medication that made him "sleep quite heavily".
The pair will be sentenced next week.