Police suspect the five people found dead in north Sydney on Saturday were all murdered in a violent home invasion.
The bodies who are believed to be a 45-year-old man and his 43-year-old wife, their sons aged 12 and nine, and the woman's 39-year-old sister, were found by a relative on Saturday morning.
They were bludgeoned to death with a blunt instrument, suffering blows to the head and upper body.
Homicide squad Detective Superintendent Geoff Beresford says an earlier suggestion of a murder-suicide has now been ruled out and it appears the five were all murdered in a targeted attack in the North Epping home.
He says a strike force has been set up to determine the circumstances of the deaths and a number of options are being explored.
He says "it's very early in the inquiry, but at this stage it appears that this particular murder does not bear the hallmarks of a typical home invasion".
"Whilst we can't totally rule out robbery, robbery does not seem to be an issue in this case," he said.
Detective Superintendent Beresford says a motive for the killings is yet to be established but he has described the slayings as brutal.
He says there has been no link established with a recent armed robbery that the father was a witness to.
A teenage daughter who was overseas visiting relatives returned to Sydney on Sunday afternoon.
The Boundary Road property remains cordoned off.
Police say post mortem examinations on the bodies will take at least two days.