He reportedly said "leave me, leave me, my son, my son".
She said police had told her petrol from a lawnmower had been found at the scene and could have been used as an accelerant.
Bruce also claimed that her sister and Millar had an on-again-off-again relationship and had been involved for about five years.
Katie Bruce had left Millar two weeks before the fire and moved in with her older sister.
Bruce claimed the relationship was "abusive" and her sister often called her for help.
She told Fairfax that her sister was "smiling again" after leaving Millar, but he "always said if he couldn't have her, then no one would and he'd be the first to take her out".
Before he died Millar allegedly told Bruce's partner he started the fire.
The officer in charge of the investigation into the fire said police could not comment as the matter had been referred to the Coroner.
"Once we have completed our enquiries, the Coroner will set an inquest date".