Mathias Pogba has been charged and detained over an alleged plot to extort money from his brother Paul, the France and Juventus midfielder.
Four other people have also been placed under formal investigation for extortion and criminal association, judicial sources told news agencies Reuters and Agence France-Presse.
Mathias Pogba's lawyer, Yassine Bouzrou, said his client is innocent.
He told French broadcaster BFMTV: "We are going to challenge this decision."
Mathias Pogba, 32, has admitted he was behind a video that appeared online last month promising "revelations" about the 29-year-old Juventus midfielder.
French prosecutors opened a judicial investigation earlier this month after former Manchester United player Paul said he was the target of extortion and threats from an organised gang.
He filed a complaint with Turin prosecutors on 16 July claiming he was the target of a 13m euro (£11.29m) blackmail plot.
A World Cup winner with France in 2018, Pogba moved to Juventus on a free transfer this summer after leaving Manchester United.