Sandra Leigh Houston showed no signs of urgency or panic as she handed her five-month-old niece to hospital staff saying the baby "seemed to be dead or dying".
Within an hour the now 38-year-old had returned to her Waterford West home, south of Brisbane, and her husband went to work.
They made no inquiries about baby Seneh and the following day, November 20, 2018, the little girl's life support was turned off.
Seneh died from significant head injuries consistent with being shaken briefly and forcibly put on the floor or a mattress.
But she also had 38 other injuries like leg and rib fractures, bruises and abrasions.
They had been inflicted at different times and must have been caused by force, a Brisbane court has been told.
Nearly a month after Seneh died Houston went voluntarily to police, saying she had to be the one responsible for the baby's death, and that it was possible she exerted too much force.
Houston pleaded guilty in the Brisbane Supreme Court on Wednesday to one count each of manslaughter, grievous bodily harm and cruelty to a child.
She has no criminal history and was not affected by alcohol or drugs at the time of the death.
Seneh's mother, who was Houston's twin, died when the baby was less than two months old.
Houston and husband Jason Michael Bowman, who together had four children under the age of six, took Seneh and her then four-year-old sister into their care.
Bowman said the baby started crying hysterically when he changed her nappy after taking a child to school on November 19, 2018, the court was told.
"It was the worst crying, he says, that he had ever seen," Justice Thomas Bradley said.
As he put her on the blankets where she slept in their bedroom Seneh went limp and was not breathing properly.
Bowman drove to the hospital with Houston carrying Seneh and accompanied by the children.
Houston handed over the child, without panic or urgency, saying she "seemed to be dead or dying".
The court heard Houston struggled with parenting before her nieces came into her care, but was later grieving her sister's death and told a psychiatrist of feeling obliged to care for the baby.
She would get irritated by Seneh not feeding, sometimes smacking her "with full force", Justice Bradley said.
In a letter to the court Houston said she felt ashamed and deeply guilty.
"You accept that from your actions you have, in your own words, 'ruined your own family forever'," Justice Bradley said.
There was no single incident in which Houston lost control, but over weeks Seneh was "so roughly treated" as to suffer broken bones in her legs.
"It would have been obvious that the baby was subject to such pain she was in need of medical care - none was sought," he added.
Houston's conduct "involved cruel and ultimately lethal mistreatment" of Seneh.
Tears streamed down Houston's face during court proceedings and her body shook as Justice Bradley handed down a head sentence of nine years behind bars.
Bowman is charged with one count of cruelty to a child under 16 and is due to have his case mentioned in the same court on July 13.
