Francesca Grillo, 35, and Elisabetta, 41, had denied spending £685,000 on credit cards belonging to the TV cook and her ex-husband.
They claimed Nigella authorised them to spend the money as she wanted to hide her drug use.
A verdict was reached on the jury's second day of deliberations at Isleworth Crown Court.
Neither of the defendants were in court to hear the verdicts, after Elisabetta was taken ill. As the jury were sent out on Friday morning, Judge Robin Johnson explained that while "neither defendant is in the dock, they are in the building".
"One of them is not feeling well, is very anxious and you will appreciate that a criminal trial is a stressful business for many people, no more stressful than for the person on trial, in this case, herself.
"I have said that the defendants can be in another room while court proceedings proceed, with the consent of the barristers."
The verdict follows an extraordinary trial during which Nigella admitted to taking cocaine and cannabis, but denied she was a drug addict. She also told the court she had turned to cocaine after being subjected to "intimate terror" by Charles.
The sisters' barrister had told the jury that they had been "caught in the crossfire" of the fallout from Nigella and Charles' bitter divorce, and that the trial was being used by the former couple to fight each other "by proxy".