PARIS (Reuters) - The sister of French Islamist militants Fabien and Jean-Michel Clain was put under formal investigation for criminal association in relation to a terrorist undertaking and remanded in custody on Saturday, a judicial source said.

Anne-Diana Clain, born in 1975, was arrested Tuesday night at Roissy international airport near Paris along with her partner, who had already been put under investigation for the same reason, and their eldest son.

The family had been expelled from Turkey, which serves as a gateway to Syria for Islamist militants from Western countries.

Fabien Clain appeared on the audio recording of Islamic State's claim of responsibility for the attacks on Paris in November 2015 that killed 130 people.

The voice of his younger brother Jean-Michel was heard behind him on the recording that was posted by Islamist State.

French authorities suspect the two brothers, who are from the Réunion island in the Indian Ocean, to be in Syria.

(Reporting by Emmanuel Jarry; Writing by Mathieu Rosemain; Editing by Alison Williams)