A co-owner of the Swiss bar ravaged by a deadly fire was charged with forgery on Friday, adding to the numerous charges she and her husband already face over the New Year disaster that killed 41 people.

Jessica and Jacques Moretti already face charges including manslaughter by negligence and arson by negligence over the blaze at their bar Le Constellation, in the Crans-Montana ski resort in the early hours of January 1.

As prosecutors questioned the French couple at an investigation hearing in Sion in southwest Switzerland, Jessica Moretti's lawyers confirmed she faced a new charge of falsifying a document.

The charge is linked to a falsified invoice believed to be central to the case, though her lawyers said she was innocent and the document was not relevant to the case.

The invoice was for the sound-insulation foam that lined the ceiling of the bar's basement, which prosecutors say ignited when champagne bottles with sparklers attached were raised into the air.

Most of those killed were teenagers, many aged 16 and under, while 115 others also injured in the blaze. 

Moretti's lawyers said their client had willingly provided an invoice to investigators for a 2015 purchase.

"This was a real purchase that was simply recorded in the accounts under a different name," they said, insisting that the charge was "clearly unrelated" to the tragedy. 

Lawyers for the victims' families saw it differently.

The invoice raises "very legitimate questions", lawyer Christophe de Galembert told reporters. "We have no answers."

'Playing the victim'

Speaking to reporters at the end of the 10-hour-long hearing, lawyer Yael Hayat said his client Jessica Moretti "sometimes feels that in reality, what she is being criticised for is surviving".

Among those present at the hearing was Laetitia Brodard-Sitre, whose 16-year-old son Arthur died in the disaster.

She accused the Morettis of "playing the victim" during the hearing.

"They take no responsibility... How do you expect us, the parents, who visit our children's graves every day, to react," she asked.

The Morettis have been questioned twice since the criminal investigation against them was opened but not since February and never together.

Lawyers for the families complained Friday's hearing was too late in the investigation, and that little had been done to guard against evidence tampering and collusion.

"This is a couple who share a household: it's the Everest of collusion," said lawyer Gilles-Antoine Hofstetter.

"There's a strong chance their versions have been coordinated."

Fourteen people are under criminal investigation in connection with the disaster, including several current and former local officials.