A woman, believed to be aged in her 60s and the deceased man’s mother-in-law, was treated at the scene by NSW Ambulance paramedics, before being taken to Westmead hospital in a critical condition.
A second woman, aged in her 40s, sustained minor injuries and was not taken to hospital.
Two thirteen-year-old girls were home at the time but were not harmed.
"There's nothing more tragic than coming to a family situation and someone is deceased. It's unacceptable,” Hawksbury Police Area Commander Detective Superintendent Jim Stewart told reporters.
"People have to be safe in their homes. You have to look after each other. It's gruesome, horrific. It's always horrible to walk into a situation like that."
Police are currently investigating the scene, and so far nobody has been charged.