I don't think that there is much point in comparing the sister's credit card bills with Nigella's though as a chunk of their spending was on their employers behalf. i image disentangling the two would be tricky. After all their limits were raised twice, so it must have been felt that there was a legitimate reason for them to be spending very large amounts of money.
Clearly things got completely out of hand, but I'd bet that was a gradual thing, a little bit here and there to start with, possibly condoned, and then as things got more chaotic the dipping got bigger and more routine.
Fraud specialist use something called the fraud triangle:
*Incentive/Pressure Pressure, such as a financial need, is the “motive” for committing the fraud. One common pressure is a gambling problem.
*Rationalization The person committing the fraud frequently rationalizes the fraud. Rationalizations may include, “I’ll pay the money back”, “They will never miss the funds”, or “They don’t pay me enough.”
*Opportunity The person committing the fraud sees an internal control weakness and, believing no one will notice if funds are taken, begins the fraud with a small amount of money. If no one notices, the amount will usually grow larger.
Not sure what the pressure might have been here, but the rationalisation and opportunity seem clear enough.