Death of man, 91, was 'family related'

An elderly man found dead with stab wounds in Sydney's southwest was the victim of family-related violence.

A 24-year-old man was under guard in hospital on Thursday after a 91-year-old man was found by NSW Police on Wednesday night at a home in Roselands.

Campsie Police Superintendent Sheridan Waldau confirmed the victim suffered stab wounds before adding the exact cause of death would be determined by a coroner.

The 24-year-old, who had recently become known to police, knew the victim and was a family member.

"We do believe it's a family related matter," Supt Waldau told reporters.

Hydrochloric acid was found "spread around the home", but not on the body, she said.

Two officers who first entered the house were overcome by fumes and had to be treated for chemical burns. Both have recovered.

"We're not sure of the circumstances of exactly what happened last night. We haven't been able to speak to the 24-year-old male," Supt Waldau said.

The detained man is undergoing treatment and could undergo surgery, for injuries sustained before his arrest, she added.

Police were initially contacted by the dead man's son, who had to be restrained by officers as the 24-year-old was taken into custody.