Jack Edwards' final action was a desperate attempt to protect his little sister from their homicidal father.

Police sources close to the investigation into last week's tragic double homicide at West Pennant Hills said the 15-year-old died trying to shield Jennifer Edwards, 13, from their estranged father John as he shot at them with handguns, which police described as "powerful".

Jack Edwards (right) tried to protect his sister Jennifer.

The Herald revealed on Wednesday that Mr Edwards was legally able to obtain a handgun permit and licence at St Mary's Indoor Shooting Centre because he was granted a Commissioner's Permit. He was given the permit despite his answers on a declaration form raising red flags that saw him refused membership at other clubs.

The Normanhurst man, who returned home and killed himself after fatally shooting his two children, was known to police and had in the past been the subject of an AVO, but did not have an "extensive history" of criminal behaviour.