French investigators suspect Anis Hanachi, described as a former jihadist fighter in the Iraq-Syria region, of complicity in the attack by his brother Ahmed.
The source said Anis would be charged on Friday.
According to Italian anti-terrorism chief Lamberto Giannini, French investigators are looking into whether Anis "indoctrinated... Ahmed and caused his radicalisation".
He was arrested in the northern Italian town of Ferrara six days after Ahmed Hanachi, a 29-year-old Tunisian, fatally stabbed the two women outside Marseille's main rail station on October 1 before being shot dead by police.
Ahmed was known to police for drug and alcohol problems with a history of petty crime. He was not on a jihadist watch list.
Two days before the attack, he was arrested for shoplifting in the eastern city of Lyon but was allowed to walk free the following day.
The French government's inspectorate general (IGA) said that decision revealed "serious faults" in the system for dealing with foreigners whose papers are not in order.
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