New South Wales police have confirmed that the sister of teenage gunman Farhad Jabar fled to Syria the day before he killed a police accountant in October, accusing a 20-year-old western Sydney man of facilitating her travel.

Milad Atai, of Guildford, was one of four men charged on Wednesday with plotting the terrorist shooting of Curtis Cheng.

Atai was also charged with being a member of a listed terrorist organisation, likely to be Islamic State, the only one of the four accused of joining the group.

Cheng, an accountant and father of two, was shot and killed as he left Parramatta police headquarters by 15-year-old Farhad Jabar, who was subsequently shot dead by police.

Police accused Atai on Wednesday of helping spirit Jabar’s sister to Syria the day before Cheng was killed, the first official confirmation she has fled to the war-torn country.

NSW police deputy commissioner Catherine Burn declined to comment on the sister’s involvement in the alleged conspiracy to kill Cheng, if any, or on her current whereabouts.

The three other men, Raban Alou, Mustafa Dirani and Talal Alameddine, were already on remand in Goulburn’s Supermax prison facing charges over 58-year-old Cheng’s killing, along with a raft of other terrorism offences.

The four were charged on Wednesday with conspiring to do an act in preparation for a terrorist attack, which carries a maximum sentence of life in prison.

Alou, 18, was charged in January with being the ringleader of a homegrown terrorist group that allegedly included Alameddine, 22, and 23-year-old Dirani. Atai was also charged on Wednesday with being a member of the unlisted terrorist group.

The Guildford teenager Alou is already accused of passing Jabar a Smith and Wesson revolver in a section of the Parramatta mosque outside the view of CCTV cameras. Alameddine is believed to have sourced the gun.

Atai, 20, had not previously been linked to the Cheng murder and was arrested in March this year after allegedly giving funds to a 16-year-old schoolgirl, to be passed onto Islamic State.

It is unclear why Atai has been charged with being a member of Islamic State rather than the local group allegedly headed by Alou.

Police will allege Atai too was involved in meetings to plot Cheng’s killing in the days before 2 October last year.

Alammeddine is facing additional charges of refusing to answer 20 questions from the Crime Commission over the murder.

The men are part of 14 people charged with alleged terrorism offences under Operation Appleby, a counter-terrorism investigation focusing on links between Mohammad Baryalei, a senior Islamic State recruiter in Syria, and those he allegedly encouraged to commit violence back in Australia.

Among them is 23-year-old Omarjan Azari, accused of agreeing to Baryalei’s orders to snatch “any random unbeliever” and “finish him, finish her [and] put the flag of the [Islamic] state in the background”.