"While no one was physically injured or killed as a consequence of the offence, the offenders nonetheless achieved their aim of creating terror in the minds of members of the public," Justice Christine Adamson said on Tuesday.

Khaled Khayat, 52, and Mahmoud Khayat, 34, were found guilty in the NSW Supreme Court earlier this year of conspiring between January and July 2017 to prepare or plan a terrorist act.

The judge jailed the older brother for 40 years and his younger sibling for 36 years with non-parole periods of 30 and 27 years respectively.

They plotted with their older brother, Tarek Khayat, who fought for Islamic State in Syria, and "the controller", an unidentified person connected with him, to blow up the plane and carry out a lethal poisonous gas attack.

A bomb, sent in a parcel by Tarek Khayat to Sydney, was hidden in a meat grinder and put into the luggage of their unsuspecting brother, Amer Khayat, who was flying to Abu Dhabi.

"Neither offender was particularly close to Amer, although each said he loved him as a brother and spoke to him when he saw him," Justice Adamson said.

"They disapproved of him because he drank, went clubbing, gambled and was gay which they regarded as bringing shame on the family."

Their bomb plan was abandoned when the luggage was found to be overweight at Sydney Airport on July 15 in 2017.

After a baggage attendant told them to rearrange Amer's luggage, Khaled Khayat removed the bomb due to fears of it being detected.

The conspiracy plainly envisaged that a large number of people would be killed, the judge said.

"If the plots had gone according to plan, no one in the aircraft carrying the bomb and no one exposed to the poisonous gas would have survived and no one would have had time to say goodbye," she said.

"The scale of the intended impact adds significantly to the gravity of the offence."

Tarek Khayat, a leader of Islamic State and a sheikh, fought in Syria with his three sons and a nephew, two of whom were killed while fighting in Syria.

Memorial messages venerating their martyrdom were sent to the Sydney brothers, while messages sent by the brothers indicated their sympathies were with the cause of Islamic State.

They "felt a family and religious obligation to help Tarek in a cause in which they all believed", the judge said.

"Tarek and the Controller were the guiding minds of the conspiracy, leaving Khaled and Mahmoud to do the requisite physical acts to bring it about in Australia."

The Sydney brothers, who maintain their innocence, had migrated to Australia, had loving families and "at least apparently" were well-integrated into life here.

"Although their sympathies were with Islamic State because of their family and religious connection, I doubt that either was prepared to become a martyr to the cause."

After more than two years in a Lebanese prison, Amer Khayat returned to Sydney after being found not guilty in September by a military court of the bomb plot.

"Amer gave evidence that Tarek, who is subject to a death sentence in Iraq, is either already dead or close to death from lung cancer," the judge said.

Mahmoud Khayat's lawyer later said he would appeal his conviction and sentence.

Australian Associated Press