Pascal Troadec and his wife Brigitte, both 49, and their 21-year-old son Sebastien and daughter Charlotte, 18, went missing on February 16th in a case that has gripped France.
Police searches at the couple's suburban home in the western city of Nantes found traces of blood from Sebastien and the parents, but not Charlotte, as well as signs of efforts to clean them up.
Pascal Troadec's sister and her husband — already quizzed by investigators at the start of the probe — were taken into custody on Sunday.
One investigation source said on Monday that the brother-in-law, who has not been named, had admitted killing the family in a row over an inheritance.
A judicial probe into murder, abduction and illegal confinement was opened last Monday after the authorities were alerted to the disappearance.
The initial suspicion fell on the son Sebastien.
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.”
Neighbours and a source close to the probe said Sebastien had had psychological problems and Pascal had suffered from depression in the past.
Sennes noted that Sebastien had been sentenced to carry out community service after being convicted in 2013, when he was a minor, for making death threats on his blog.
But classmates of Sebastien's interviewed by French media described a quiet, friendly youth, with one saying he was “always ready to help out”.
Bloodstains were found at the house in Orvault, including on Sebastien's cellphone and on his mother's watch, as well as efforts to wipe some of them away.
Investigators returned to the house on Wednesday with “very sophisticated” scientific and technical equipment to search for further clues, Nantes prosecutor Pierre Sennes said.
Wearing white forensic suits, they were seen entering and leaving the mystery house, where a police security zone was set up.
Traces of blood from the father, who works for a company that makes shop signs, his wife and their son have been identified but so far there are no signs of the daughter's blood, Sennes said.
He said earlier that investigators had been unable to find Sebastien's car. The parents' two cars have been sequestered.
No toothbrushes or hairbrushes were found in the house, and the beds had been stripped, with some sheets drying on an indoor rack.
Food was going off in the refrigerator and there were dishes in the sink, according to Sennes, who said it was as “if the life of the house was frozen in time”.
Sebastien's cellphone was the last to be switched off, Sennes said.
On February 16, the mother and daughter reported the fraudulent use of Charlotte's bank card to pay for online video gaming, Sennes said Wednesday.