The sister and brother-in-law of Frenchman Pascal Troadec who has been missing along with his wife and two children since mid-February were remanded in custody on Sunday, sources close to the case said.
The couple had already been questioned by police at the beginning of the inquiry into the mysterious disappearance of the Troadec family, the sources said confirming reports in a local newspaper.
Nantes prosecutor Pierre Sennes confirmed that two people were in custody but declined to give their identities.
The two were detained in the Brittany city of Brest, the region the Troadecs came from and where several personal items belonging to members of the family have been found.
Pascal Troadec, his wife Brigitte, and their son Sebastien, 21, and daughter Charlotte, 18, went missing on February 16.
In searches at the parents' home in a suburb of Nantes, police discovered Sebastien's cellphone and his mother's watch covered with blood.
Traces of blood from Sebastien and the parents, but not Charlotte, were found throughout the house, as well as signs of efforts to clean them up.
Police said in a bulletin dated February 24 that Sebastien was suspected of "putting in place a macabre plan aimed at snuffing out the members of his family and maybe himself".
Investigators have found Sebastien's car and a jogger discovered items belonging to his sister Charlotte in a forested area three hours by car from where she was last seen.
A judicial probe into murder, abduction and illegal confinement was opened on Monday