The women were detained after Iraqi fighters ousted the jihadists from Mosul last July, one source said, confirming a report on RMC radio.
One 28-year-old woman left in 2015 for the group's "caliphate" stretching over parts of Syria and Iraq along with her husband, who has reportedly been killed.
She is being detained with her daughter, who was born after their arrival.
"We don't know what exactly she is accused of, what her detention conditions are like and whether she is being allowed the means to defend herself," said the woman's lawyer, Martin Pradel.
He said he had received "no response" from France's foreign ministry on the case. The Red Cross has been his only source of information, he added.
A second woman, a 27-year-old named as Melina, also left for the region in 2015, and is being held with her baby. Her three older children have been returned to France.
"We expect France, if Melina is sentenced to death, to mobilise with the same intensity it has for other French citizens sentenced to death, in particular Serge Atlaoui," said her lawyers, William Bourdon and Vincent Brengarth.
French diplomats have waged an intense campaign to free Atlaoui, who is being held in Indonesia and facing the death penalty on drug trafficking charges.
But government officials have said French fighters arrested in Syria and Iraq should be tried there if they can be guaranteed a fair trial.
Defence minister Florence Parly said Sunday that "we can't be naive" regarding French citizens who left to join IS.
"When they are caught by local authorities, as far as possible they should be tried by these local authorities," she told France 3 television.
On Sunday, an Iraqi court sentenced a German woman to death by hanging after finding her guilty of belonging to IS, the first such sentence in a case involving a European woman.
Around 40 French nationals, both men and women, are currently in detention camps or prisons in Syria and Iraq, with about 20 children, a source has said.
- with AFP