It is a terrible story but it throws a grim light on the terrors of the Syrian war. It is told at first in a calm, precise voice by Nusair Mahla, a middle-aged government employee, until he finally has to choke back tears as he speaks of the last moments of his sister Maysoun Hala and her husband Nizar along with their two children, Karim and Bishr. He says that many other Syrians have suffered similar tragedies, but in few cases is it known so precisely what the victims themselves thought about their fate.

Nusair, a neatly dressed man in a brown suit, says the first he knew about his sister's family being in danger was an early morning phone call. He recalls it came after 6.30am and was from neighbours who said that insurgents, whom he invariably calls "terrorists", had entered the industrial town of Adra 12 miles north of Damascus and were taking hostages. This happened on 11 December when fighters from the al-Qa'ida affiliate Jabhat al-Nusra and the Islamic Front, another jihadi group, had captured the main employees' residential complex at Adra using an old sewer to outflank government forces.

Nusair recalls: "I immediately called my sister and told her to get out and come to my house in Mezze." Nizar worked as a public relations specialist for the state oil company while Maysoun had qualified as an engineer at Damascus University and was a housing manager at Adra. As state employees they were at risk of being killed by jihadi rebels, but what made their execution certain was that, though very secular in life-style, they belonged to the Alawite sect, a variant of Shi'ism to which President Bashar al-Assad and many of the Syrian ruling elite belong.

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In the event, the jihadis who had taken Adra believed that state employment or membership of any religious minority – Alawite, Christian, Druze – was enough to merit death. Maysoun told her brother that she didn't dare follow his advice to leave her apartment building because the rebels were "in front of the door of the building and they are they are also on the roof tops". Even so, Nusair suggested she go with the two children, Karim, 16, and Bishr, five, and maybe the jihadis would let them pass.

She answered that "they look so terrifying and I am afraid. I was looking out the window and I saw the terrorists killed one of the NDF [pro-government National Defence Force militia] with a big knife". Maysoun explained to Nusair that she and Nizar planned to try to wedge the door of their apartment shut. But if this failed and the jihadis broke in, then the whole family had taken a momentous resolution: rather than face torture and inevitable death at the hands of Jabhat al-Nusra, they would die as a family by detonating grenades that Nizar had somehow acquired (he does not seem to have had a gun).

The rest of the day was chaos, according to Nusair. All the relatives of the trapped family were calling them on landlines and mobiles to try to comfort them. At about midnight, Nusair's 25-year-old son William, who lives in Aleppo, called his aunt and asked about the situation. Maysoun replied: "They are trying to get into the house." William heard two gunshots. Maysoun repeated that if the rebels got in the family would blow themselves up. Then she cried out: "They are inside the house, William, they are inside the house! We should say goodbye. Please forgive us." Then he heard an explosion.

Nusair breaks off telling the story and puts his hands over his eyes as he tries to suppress his sobs. After a few moments he goes on to say that William phoned him and said: "They are now in the hands of God." Nusair called the landline and mobiles of the family in the apartment in Adra and there was no reply. He stayed at home with his own family and Damascus and told them "we can't do anything. They are now martyrs."

Nusair gives some background about Nizar and Maysoun: "They were a wonderful family. They were like a small democracy. Anything they wanted to do they discussed and, even if five-year-old Bishr was against doing something, they didn't do it." Nizar was very secular – "I used to call him a secular extremist" – and pictures of the family together show Karim with long hair and his mother with her hair dyed blond and without a headscarf. She had been working hard trying to get ready housing for refugees from Douma, a rebel stronghold not far from Adra. Much of this emergency accommodation was in a half-built residential complex with no glass in the windows and no furniture. Nusair, evidently very close to his sister, says: "I used to call her every night and she would say, 'I am so tired preparing these houses that aren't ready for people to live in'."

The story does not end with the explosion and the apparent death of the family. At 3am the next day, Nusair got a call from Nizar's brother who said: "Nizar just called me but the line was cut." Nusair immediately called the Adra apartment on the landline and it was answered. He asked: "What happened, Nizar?" Nizar replied in a slurred voice as if in pain: "Bishr died and Maysoun and Karim are badly injured and bleeding. They are not moving. It is too late for me but please try to do something for them." Nusair talked and tried to say encouraging things but, he says, "finally the phone must have fallen from his hand. Those were the last words I heard him say."

As to what happened next, Nusair says the details are unclear. Some neighbours in Adra said in the single telephone call they were able to make that they had gone to the apartment and found Nizar, Karim and Bishr dead, but that Maysoun was alive, although her leg was severed. They took her to their apartment.

Then comes a final horrible twist to the story. Nusair's daughter Senna is at school in Damascus. Some of the children whose parents were able to escape from Adra are now being taught there. She asked one of them if he knew Maysoun. He said: "She was the woman with one leg cut off they [the insurgents] dragged behind a car." Nusair says that Senna fainted as soon as she heard this.

The cruelties of the Syrian civil war get worse by the week. Each side belittles its own atrocities and claims them as retaliation for something even worse done by their enemies. Nusair explains why his sister, brother-in-law and their children decided to kill themselves: he says they believed that "the jihadis would kill the youngest child in front of the mother and rape the mother in front of the husband. They would torture the men and then kill them all anyway. Better to die by their own hand."

Accounts of what happened to the rest of the population of Adra are confused. I spoke to some of the 5,000 refugees who had been allowed to leave by Jabhat al-Nusra and the Islamic Front on 30 December and some of whom are now squatting in a giant cement factory. They said the jihadis had ordered them to their basements and had kept them there. The number singled out for execution is put at between 32 and 80. There are accounts of the doctor in the local clinic, a Christian known locally as Dr George, being decapitated. Bakery workers who resisted their machinery being taken away were roasted in their own oven. Jabhat al-Nusra and Islamic Front fighters went from house to house with a list of names and none of those taken away then has been since. This includes the head of the legal department at the Information Ministry who disappeared with his wife and daughter and whose phone is now being answered by a man saying he belongs to Jabat al-Nusra.

The story of how Nizar and Haysoun's family died together is well known in Damascus. The many Syrians who work for the government or belong to minorities wonder what they would do if the jihadis were at their door. It is tragedies like this one that provide the fuel for an ever more savage civil war.

Ceasefire breached

A ceasefire arranged in Homs to allow the delivery of aid and the evacuation of women, children and the elderly was breached yesterday as mortars were fired across the besieged city. Both sides accused each other of breaking the fragile peace.

Four members of the Syrian Arab Red Crescent were reportedly wounded by rebel fighters as aid workers tried to deliver humanitarian supplies. State media said the Red Crescent members came under fire from "armed terrorist groups".

The regional Arab television station Al Mayadeen reported that it was hoped that trucks carrying food, water and medicine for up to 3,000 residents would still be able to enter the city yesterday.

The opposition Shaam News Network, which blamed President Bashar al-Assad for the attacks, said the mortar fire had coincided with the entry of two UN cars into the besieged districts. Mortars were reported to have targeted an area of Homs near to where negotiations over the aid operations were taking place.

Oscar Williams