Francesca Grillo, 35, and her sister Elisabetta, 41, are alleged to have spent £685,000 on credit cards belonging to the TV cook and her millionaire art dealer ex-husband, who divorced in July.
Anthony Metzer QC, defence counsel for Elisabetta, said she had been caught up in the “cross-fire” between the former couple.
In his closing speech he asked the jury at Isleworth Crown Court, west London: “Could it be Mr Saatchi was using this as a way to attack Ms Lawson by proxy? By turning on one of her most trusted and loved people?”
He added: “As his relationship with Ms Lawson started to unravel and he lost control of her, he looked for a place to put his hurt and anger.
“The extravagant way Ms Lawson kept her family in his money was now a legitimate place for him to exert his feelings.”
Mr Metzer asked why Mr Saatchi did not carry out a full investigation into all of his staff.
“He knew Lisa [Elisabetta] was a soft underbelly for attacking Ms Lawson,” he said.
She was “caught in collateral cross-fire” between Mr Saatchi and Ms Lawson, like the child of divorcing parents.
The sisters have told the court they saw evidence that Ms Lawson used drugs on a regular basis.
Francesca told the jury yesterday she would “frequently” find rolled-up banknotes with white powder on them in handbags belonging to the food writer. In her evidence to the court, Ms Lawson said she used cocaine on only one occasion following the death of her first husband John Diamond, and had taken it on a small number of occasions with him.
Mr Metzer asked if that was “credible”, given the addictive nature of drugs. He said Ms Lawson “carefully honed” her evidence and gave a “rehearsed speech”.
He said: “In reality, what choice did she have? She faced compelling evidence from many sources of sustained drug use over 10 years.”
The Grillos, of Bayswater, west London, each deny a single count of committing fraud by using a company credit card for personal gain between January 1 2008 and December 31 last year.