He claimed it had been at least 10 hours, she insisted it was only two to three.
"He [Karlos] was grizzly and crying, you couldn't handle the situation ... it was you who assaulted the baby while Mr Roberts was away from the address, that is what happened, isn't it?" Simpkins demanded.
Stephens categorically rejected the allegation he levelled at her.
Simpkins continued his questioning by saying she needed to blame Roberts otherwise the focus for Karlos' death would have been on her.
Again Stephens responded sharply saying "absolutely not".
She denied she'd once asked Roberts to pick the boys up from her Te Ngae flat because she thought they were possessed or that she was having frightening thoughts about harming her babies.
Challenged that someone had to break into a room to get to her finding her with her hands around her five-year-old son's neck, Stephens defended herself, saying she was holding him to prevent him being taken from her.
Asked when she had last seen her surviving twin, Hosea, Stephens said it had been about 2015 when she bumped into him while Christmas shopping at The Warehouse. He was with Roberts' former wife.
Re-examined by the prosecutor Amanda Gordon, Stephens said she hadn't attended her baby's tangi in Te Whaiti because of miscommunication - she'd thought she was going with her mother who, in turn, thought her daughter was being taken there by someone else.
The jury has begun watching a DVD recording of a conversation a trained child interviewer had with another of Stephens' sons, then five.
He talked of going to Kraut's house with his mother and baby brothers. The court has heard Roberts was frequently known by that name.
Earlier in the day Stephens told the jury Roberts had instructed her to say baby Carlos had died from cot death.
She admitted giving two versions of events surrounding the infant's death to police. who reopened their investigations into it last year.
The trial is proceeding.