A family in Tripoli say they are still in shock over allegations that three brothers plotted to blow up an Etihad flight departing Sydney.

Key points: Khaled and Mahmoud Khayat will face trial next year, accused of planting explosives in their brother's luggage to blow up a plane

Khaled and Mahmoud Khayat will face trial next year, accused of planting explosives in their brother's luggage to blow up a plane Waled said his older brother Khaled went to Australia in the mid-80s to make a living, and said if he were a terrorist, he would not have left Lebanon

Waled said his older brother Khaled went to Australia in the mid-80s to make a living, and said if he were a terrorist, he would not have left Lebanon Waled said his brothers in Australia were not extremists and had no interest in groups like IS

Speaking publicly for the first time, the brothers of the accused men say they do not understand how the allegations about the July 2017 plot could be true.

"I feel like I've aged with this," said Waled Khayat, speaking from his home in Lebanon's second city of Tripoli.

"I've started taking anxiety pills because of this.

"I don't understand what happened with them. What is this story?"

Khaled and Mahmoud Khayat will face trial in the NSW District Court next year, accused of planting explosives in the luggage of another brother, Amer, in order to blow up a plane in July last year.

The Australian Federal Police said Amer did not know anything about the explosives, which were allegedly removed from his bags at check-in by his brothers because his bags were overweight.

Amer made it to Lebanon to stay with Waled and his family.

"He brought three trousers for my children, a pair of shoes for me, a pair of tennis shoes for my nephew. And hand bags for my sisters. And he gave to each of us $100," said Waled.

Waled said they were stunned when they watched news broadcasts of the brothers being arrested in Australia. A week later, Lebanese authorities came for Amer.

He is still in custody more than a year later, and is being investigated by a military tribunal.

Fadi Khayat said some of the evidence presented made no sense. ( ABC News: Aaron Hollett )

Fadi Khayat, who also lives in Tripoli, said the case against his brother was thin.

At the military tribunal on Tuesday, authorities presented a series of photos said to implicate him: nine screenshots of news reports about terrorism, and an image of a letter supposedly written by Amer to his mother which states in part, "I agree I was mean to you and did very wrong".

Amer Khayat said the screenshots were not taken by him.

Fadi Khayat said the letter to their mother made no sense, since she died in 2010.

"Someone must have played with his phone. That's for sure as it's impossible."

Tarek Khayat accused of being IS commander

Waled said his older brother Khaled went to Australia in the mid-80s in the midst of the Lebanese civil war to find work in a more peaceful country.

"Khaled has been to Mecca, so he knows right from wrong. Khaled would have never left Lebanon amidst the civil war to travel to Australia. Had he wanted to participate in a war he wouldn't have left Lebanon to go there.

"He was there to make a living. He wants to down a plane? It's not logical. If he were a terrorist, he wouldn't have left Lebanon in 1986."

The Khayat family as children: L-R Fadi, Amer, Tarek (at rear), Iman (the girl) and Waled. ( Supplied )

Waled said his brothers in Australia were not extremists and had no interest in groups like the Islamic State (ISIS) group — referred to in Lebanon as Daesh.

"Khaled, Mahmoud and Amer never spoke about Daesh. They were never part of Daesh. Amer even thought that Daesh was a person. That's what he thought. He didn't know about the organisation and thought it was the name of a man."

There are seven surviving brothers in the Khayat family and it is alleged one of them, Tarek, knows a lot about ISIS — he was picked up in Iraq earlier this year, accused of being an ISIS commander.

Waleed said the rest of the family should not be judged by Tarek's actions.

"I don't want to talk about Tarek. I don't care. He chose his own path. But Khaled and Mahmoud haven't chosen what is happening to them.

"Of course Tarek's situation had a terrible impact on our family. My sons have grown up now. I fear that they could be arrested because of him."

Waled visited Amer in prison in Beirut earlier this week. The men's sister had died the previous day.

"Amer wishes to go out soon. And all of a sudden he starts crying. We visited him on Monday, one day before his hearing, he burst out crying because he lost his sister and he was in jail. And he says she got the cancer because of what's happening to her brothers."

The Khayat family's problems are not going away any time soon: the ISIS brother faces execution, as does Amer in Lebanon if he is charged and convicted.

The two brothers in Australia go to trial in the NSW Supreme Court next year.