Hanachi, 29, attacked two women at Marseille's Saint-Charles train station on October 1 before being shot dead by troops.
On Tuesday, Tunisia said it had released two of his siblings who had been questioned by anti-terror investigators.
Moez and Amina Hanachi, who both live in Tunisia's northeastern region of Bizerte, had been detained on Friday, but had "nothing to do" with their brother's attack in France, prosecution spokesman Sofiene Sliti said.
Sliti said two of the attacker's other brothers, Anouar and Anis, had been targeted in a 2014 "terrorism" investigation.
Anouar was released, while Anis remained on the run.
But Swiss police on Tuesday announced the arrest of a Tunisian couple in connection with the Marseille knife attack, and a French security source close to the investigation confirmed that the man was Anouar Hanachi.
Swiss police would not confirm the identities of either the man or woman, but said the man was "known to foreign police services for his links to jihadist terrorist movements".
"His role in the Marseille attack, if he had one, is not yet clear," it said.
Federal police spokeswoman Cathy Maret separately told AFP that, in accordance with Swiss law, the detained husband and wife are due to be repatriated to Tunisia.
But that process will take several weeks at minimum and could be altered if another jurisdiction requests their extradition.
Anis Hanachi was also arrested on Saturday night in Italy after French authorities issued an international arrest warrant.
French investigators suspect him of complicity in the Marseilles attack, and he is expected to be extradited to France.
Italian authorities said their French counterparts had indicated that Anis had fought for jihadist groups in Syria and Iraq.
Attacker Ahmed Hanachi, the second-youngest of five siblings, was not known to attend any mosque, but was known to the police for drug and alcohol problems.
The Islamic State group claimed responsibility for his actions, but French investigators have not found any evidence linking the attack to the jihadists.
Relatives have said the Marseille attacker had issues with drug addiction but was not particularly religious.
Sliti, the Tunisian prosecution spokesman, said Tunisian authorities had opened a new investigation into Anouar on Monday over suspicions of supporting "terrorism", following comments posted on Facebook.
But Tunisian authorities said they have not so far established anything to link Anouar or Anis Hanachi to the Marseille killings.
Childhood friends have said that Ahmed spent several months in Tunisia in summer 2016 as he tried, with Anouar's help, to beat his addictions.
Their father Noureddine told AFP he struggled to believe Ahmed had been radicalised but that his son "may have been under the influence of drugs" when he carried out the attack.
He said he had heard no news from his sons in Europe for two months.
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