"If my brother is behind the attack, I say to him 'You dishonour us'," Abdelkader Amri told AFP on Thursday at a poor village in central Tunisia.
He spoke as his brother was on the run after Monday's attack on a Christmas market, when a large truck ploughed through a crowd, killing 11 people.
A 12th victim, the hijacked truck's Polish driver, was found shot in the cab.
Prosecutors in Germany have issued a Europe-wide wanted notice for 24-year-old Anis Amri, offering a €100,000 reward for information leading to his arrest and warning he "could be violent and armed".
On Thursday, his brothers Walid and Abdelkader were still reeling at the news.
Speaking outside their home in the town of Oueslatia, some 50 kilometres from Kairouan in eastern Tunisia, Abdelkader told AFP of his brother's departure from the North African country more than five years ago.
The youngest of nine siblings, with divorced parents and in difficult social circumstances, Amri left the country illegally in March 2011 by sea for the Italian island of Lampedusa.
He was fleeing a court conviction and a sentence of four years handed down in absentia for robbery and burglary, Abdelkader said.
A security official in the area confirmed this information.
'He had no future'
"Anis left to get away from misery - he had no future in Tunisia and wanted at all costs to improve the family's financial situation. We live below the poverty line, like most families in Oueslatia," Abdelkader said, his eyes red.
His brother Walid added: "He lived like all young people - he drank, he didn't go to prayers or anything."
Once in Italy, Anis Amri found himself in a detention centre along with other illegal migrants.
But he soon found himself in trouble: he was sentenced to four years in jail for setting fire to a building, Abdelkader said.
"In 2015 he had made it to Germany and was trying to sort himself out. He worked as an agricultural labourer and things like that," he said.
"He'd contact us on Facebook, saying he wanted to come back to Tunisia but that he had to earn some money to buy his own car and start his own business."
According to Walid, "10 days before the attack he told us he was counting on God to get him home in January."
Abdelkader said their brother "was laughing and joking with us. There was no sign he had been radicalised. I'm sure he can't have done this, that's not why he emigrated. May God reveal the truth," he said before bursting into tears, remembering that Thursday was Anis's birthday.
"I want to wish him happy birthday," he added.
Outside the house, their red-faced mother urged curious onlookers to go away.
"Please understand," she pleaded with them. "Leave us alone with our misfortune."
Risking 'the same fate'
On Thursday, the suspect's father Mustapha, as on any other day, was taking his cart around town looking for clients to transport their goods.
"Look at my father with his cart, working at that age," Abdelkader told AFP. "Here in Oueslatia, if you don't work you die of hunger."
His friend Faouzi could not contain his anger.
"Tell (President) Beji Caid Essebsi to look after the country's young people!" he cried.
"We've had enough of being ignored. We've rotted in prison! All the young people in Oueslatia risk the same fate as Anis," he warned.
Monday's attack in the German capital was claimed by the Isis terror group, and according to a security official in Oueslatia, Isis recruited the suspect in Italy.
"These cursed sons who damage the country," headlined the French-language daily paper La Presse on Thursday.
"There's no point in trying to hide the fact: every time a terrorist attack takes place in the world, the people of Tunisia hold their breath," it said.
"Because many times a cursed son has been responsible for carrying out a major attack."
Walid Anis believes in his brother's innocence, despite the belief in Germany that he carried out the attack.
A temporary residence permit believed to belong to Amri was found in the cab of the 40-tonne lorry, and Germany's interior minister said investigators had also found his fingerprints inside.
"We denounce the accusations against my brother," Walid told AFP. "We know him well. He has done nothing!"