She said the way her sister had been treated was 'absolutely diabolical'.
Cafferkey's condition worsened and the next day she was taken by ambulance to Glasgow's Queen Elizabeth University Hospital. On Saturday an emergency military flight took her to London for treatment at the Royal Free Hospital.
She had contracted the virus last December while treating patients in Sierra Leone, West Africa. The NHS nurse nearly died during three weeks spent in the Royal Free before she was given the all-clear in January.
A spokesman for the Royal Free said: 'We are sad to announce that Pauline Cafferkey's condition has deteriorated and she is now critically ill.'
Public health officials in Scotland have identified 58 people who were in close contact with the nurse shortly before she fell ill, with 40 offered vaccinations as a precaution.
Twenty-five accepted the vaccine while 15 have declined or were unable to receive it due to medical conditions.
Officials insisted children at a primary school Cafferkey visited hours before she fell ill were not at risk and Samantha Cameron, who met the nurse at Downing Street six days earlier, was also not said to be in danger.
Three British nurses fell ill with Ebola at the peak of the crisis in West Africa. But Cafferkey was believed to have made a full recovery, with aid worker William Pooley, 30, from Suffolk, and Army reserve nurse Anna Cross, 26, from Cambridge.
Experts have begun to realise that, in some patients, the virus can linger in the body in pockets of fluid - such as breast milk or behind the eye. It can then transfer back into the blood stream and reactivate.
The only other case of this kind was American doctor Ian Crozier, who had a relapse of Ebola last year. The virus had even changed the colour of his eye.
But Dr Crozier recovered - and last week scientists had expected Cafferkey to do the same, because she had built up a strong immune response to the virus.
Dr Nathalie MacDermott, of Imperial College London, said: 'This is an unprecedented situation in medical terms and an uncertain time for Pauline and her family. There is still currently no proven effective treatment for Ebola virus disease that would be known to prevent or manage secondary recurrence.'
Dr Ben Neuman, a virologist at the University of Reading, added: 'We don't know what Ebola looks like the second time around or after a big relapse like this.
'Right now, she has had this virus about three times as long as the other previous record-holder. It is bad news, but it is not hopeless news. As long as she is still there, and as long as she is still fighting, there is a chance.'
- Daily Mail
