Penelope Fillon spent Tuesday being questioned by magistrates.
She is accused of complicity in misappropriation of assets in relation to her allegedly fictional employment as a parliamentary assistant to her husband and his successor and at a magazine.
"When this case is approached calmly and with respect for the principles of law, I am convinced the innocence of Penelope and François Fillon will be recognised," commented her lawyer Pierre Cornut-Gentille.
He accused the magistrates of rushing the case through at "exceptional speed".
François Fillon already charged
François Fillon was placed under formal investigation earlier this month.
He is accused of paying hundreds of thousands of euros to his wife and children for work they did not do.
The pair deny any wrongdoing.
Though a lawmaker employing a family member is not illegal in France, Penelope Fillon is accused of doing little for the 680,000 euros she received in salary over a number of years.
Fillon has claimed that he is the victim of a plot inspired by President François Hollande.
Socialist questioned over daughters
The new blow comes less than four weeks before French voters go to the polls in a two-stage election on 23 April and 7 May.
Socialist Bruno Le Roux was also questioned on Tuesday in a similar case involving the employment of his two daughters as parliamentary assistants.Le Roux stood down as interior minister within 24 hours of the story breaking.