Abdelkader Merah was however cleared of having a direct hand in his brother Mohamed's killing of three soldiers and attack on a Jewish school, where he gunned down a rabbi, two of the rabbi's children, aged three and five, and an eight-year-old girl.
The trial was the first arising out of a wave of violence by mostly homegrown radical Islamists that has claimed the lives of more than 240 people in France in the past five years.
Mohamed Merah's March 2012 attack on Ozar Hatorah school, which he carried out in the name of Al-Qaeda, was the deadliest on Jews in France in three decades.
Over the course of his eight-day killing spree, the 23-year-old also shot dead three soldiers in the garrison town of Montauban before being killed by police after a 32-hour siege at his home.
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His 35-year-old brother and mentor Abdelkader, who had nurtured his interest in jihad and defended the killings, was arrested shortly afterwards on charges of complicity.
But while admitting to having been present when his brother stole the scooter he used in the attacks, Abdelkader denied any knowledge of his intentions.
His conviction on the separate charge of being part of a terrorist conspiracy was seen nonetheless by the victim's families as a victory.
"Justice has been served," Patrick Klugman, lawyer for the family of slain rabbi Jonathan Sandler said.
A second defendant, Fettah Malki, who supplied Mohamed Merah with a machine gun and a bullet-proof vest, was given a 14-year sentence after also being found guilty of involvement in a terrorist conspiracy.
From petty crime to terrorism
The trial lifted the lid on a dysfunctional family living on the margins of society in the high-rise Toulouse suburb of Les Izards.
Three of the five children born to Algerian immigrant parents, who later divorced, came under the spell of radical Islamists.
Both Abdelkader and Mohamed spent time in prison for acts of delinquency -- an experience that radicalised the younger Merah and left him thirsting for revenge against France.
In 2011, he travelled to the lawless tribal regions of Pakistan near the border with Afghanistan to join the Qaeda-affiliated Jund al-Khalifa.
Returning to France, he was questioned by intelligence services but insisted his trip had been solely for tourism.
Prosecutors had presented Abdelkader as the real brains behind the attacks and called for him to be given life in prison, without possibility of parole for 22 years.
But Merah's lawyers urged the jury not to make him a scapegoat for his brother's crimes to satisfy the public desire for a conviction.