The girls were abducted by their father, Matthias, from their home in Saint-Sulpice, near Lausanne, on January 30th, 2011.
Four days later, Matthias committed suicide in Italy by throwing himself under a train.
Since then, investigators have failed to find any trace of the missing girls despite extensive search efforts in several countries.
The father of the children, a 43-year-old Swiss-Canadian engineer employed by cigarette maker Philip Morris, had been separated from their mother Irina Lucidi for several months.
On the day of the twins’ disappearance, Matthias was supposed to have returned them to their mother after looking after them for a weekend.
Instead, he drove them in a car to France and his movements were subsequently traced to Marseille, Toulon, Corsica and several Italian cities.
A week after the father’s suicide, Lucidi received a shocking message from her husband. “They rest in peace. They did not suffer.”
In her most recent interview in October, Lucidi said she wanted to find her daughters “whatever the cost,” Le Matin newspaper reported.
Pascal Gilliéron, prosecutor for the canton of Vaud, said no evidence has surfaced showing where the girls might be.
“The Swiss trail, like the French, Corsican and Italian remain open and the closure of the criminal enquiry is not yet on the cards,” Gilliéron told Le Matin.
But Lucidi has expressed concerns about the lack of progress.
Last autumn Lucidi established a foundation for missing children, which runs a multilingual website (missingchildren.ch) with information in English.
The website states that Lucidi’s dramatic experience revealed to her “that certain elements and operations regarding the search for missing children need to be improved”.
On Monday, the foundation, aided by sponsors, will launch a national emergency hotline on its website, 0848 116 000. The hotline will be available 24 hours a day, seven days a week “for those who have to face . . . a case of disappearance of a minor.”