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 art dealer ex-husband, who were divorced in July. They deny the charge.
Anthony Metzer QC, defence counsel for Elisabetta, 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 said Elisabetta was "caught in collateral cross-fire" between Mr Saatchi and Ms Lawson, like the child of divorcing parents.
Karina Arden, defending Francesca, said: "Both of these people, Ms Lawson and Mr Saatchi, had their own agendas when they came to court to give evidence."
In her earlier evidence to the court, Ms Lawson said she had used cocaine.
Judge Robin Johnson yesterday told the jury: "You should consider if there is any weight to the argument that the defence made, that Nigella Lawson gave permission for this expenditure because she was worried about being shopped or exposed about this drug taking, either to Mr Saatchi or to the authorities."