Laura Joyce, 37, spent two nights in cells on the holiday island of Mallorca after being arrested on Tuesday.
She looked drawn as she arrived for the hearing.
Her mother Debbie Cameron, 59, was also held but was released on Wednesday after one night in custody.
The alleged fraud, said to be worth up to £9million, centres on claims made last year by tourists who had stayed at hotels run by three resort companies.
Hundreds of British holidaymakers are said to have been persuaded into fabricating or embellishing symptoms of gastric illness to gain compensation.
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The Club Mac hotel group alone is challenging 273 applications for a total of £4million compensation involving 797 holidaymakers.
Two other women and two men joined Joyce in court in the Balaeric island’s capital Palma yesterday.
They faced questioning by an investigating magistrate co-ordinating a long-running probe by the Guardia Civil, Spain’s national police force.
The five suspects, who are all British, were later freed but remain under investigation, an official source said.
Joyce’s passport was kept by the court, the source added.
Yesterday’s hearing took place in private. Public and press were barred from the court – normal practice for Spain where only full trials are held in public.
Mrs Cameron and her daughter were arrested at their luxury family villa in Bendinat. It is understood both women deny any wrong-doing.
A source said that Mrs Cameron has been released on condition she cooperates with the investigation.
“It is up to the judge now as to whether he takes things any further,” the source added.
Her 60-year-old brother Gary Grimstead, of Hornchurch, Essex, said: “Debbie says 100 per cent she hasn’t done anything wrong and it’s all a confusion.
“She’s back at home and is OK. They let her go.
“But all we are concerned about is Laura. She is due to give birth in the next month.”
East London-born Mrs Cameron moved to Mallorca more than 25 years ago.
She recently described her life on the sunshine island as “fantastic”.