TWO of the four Sydney brothers recruited to the Islamic State terrorist group have been revealed to be “obese” and “unfit”, and are unlikely to be allowed to fight.

A Sydney mother has been devastated by the news that her four sons — aged 17, 23, 25 and 28 — have been lost to the barbaric jihadist group.

She was shocked to receive a text from one of her boys on Friday, a day before they were due to return from a holiday in Thailand.

“We made it to Bilad al-Sham (a region in Syria). We will see you in Paradise,” the text read.

Muslim community leader and friend of the family Jamal Rifi said they had no idea their sons had been radicalised.

“From my interaction from the mother, father and family, they are devastated,” he told news.com.au.

“I see this mother grieving the potential loss of her kids, not being aware of what’s being planned by them.”

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Dr Rifi confirmed that two of the brothers were unfit and unlikely to be helpful to the extremist army.

“Two of them, they are quite obese; they are not good foot soldiers. I mean, they are over 140kg and people who are going to see them there, they are going to realise, what are they going to do with them? ‘You are going to eat our food and you can’t even run on the field?’” Dr Rifi told Sydney radio station 2UE in an interview this morning.

Sydney Muslim leaders say two of four brothers who have fled to join ISIS are "quite obese", and possibly won't be allowed to fight. — 3AW Melbourne (@3AW693) November 16, 2014

Dr Rifi did not know the sons personally, but he said the mother described them as “good, simple boys”.

“They had never been in trouble, they were doing the right things, educated well, behaving themselves. This is totally out of character,” Dr Rifi told news.com.au.

He said it was up to the whole community to learn from the tragic turn of events.

“There has to be total outrage in the community and society about what has happened, and the fact that these Australian kids have been stolen from their mother. We no longer want our kids to be lost to these barbaric recruiters,” he said.

When the mother first received the text message, she thought it was a cruel hoax.

But now, the heartbreaking reality is that four of her sons have joined more than 60 other Australians fighting for IS in Syria and Iraq.

The mother was scared by the text and deleted it but, hoping the whole thing was a joke, she sent her fifth son to Sydney Airport on Saturday to pick up her four boys. But they never arrived.

The sons had told her that they had won a “free holiday” to Thailand.

They are believed to have arrived in Bangkok and then transferred to Turkey, before making their way to Syria.

The mother has pleaded with them to come home.

“My sons, you are dear to us,” she said in a statement.

“You are good at heart. You have done nothing wrong. Come back. We love you.”

The brothers all lived at home in Sydney’s western suburbs with their Lebanese-born mother, their father, who works as a taxi driver, and their teenage sister.

It is understood the youngest was studying for his HSC at a public high school and the older brothers had stable jobs before they left.

Unlike other Australians recruited to IS, the siblings were not known to authorities and had not been on any watch lists.

Dr Rifi said the brothers were religious, but not “hard line or radical”.

The family has passed on all the information they have to counterterrorism authorities.

News.com.au has approached Immigration Minister Scott Morrison for comment.