SAO PAULO/TOKYO (Reuters) - Nissan Motor Co 7201.T has filed a court claim against Carlos Ghosn's sister that references alleged "unjust enrichment", escalating a battle being waged in Brazilian courts between the car maker and its once-celebrated former chairman.

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A court docket dated Dec. 11 that was seen by Reuters showed the Japanese automaker filed a claim against Claudine Bichara de Oliveira. The docket showed Nissan as the claimant and Oliveira as the defendant, with the subject title “unjust enrichment”. Further details were not immediately available.

Nissan said the claim, which sought a stay on the statute of limitations on a document created three years earlier, would allow it the option of pursuing further legal action.

“At this point, this is just a notice to protect Nissan’s interest from the statute of limitation,” the company said in an emailed response, when asked about the claim. “It is not specifically a request for a judgment against Oliveira.”

Nissan declined to elaborate on the document, or on the allegations against Oliveira. A representative for the Ghosn family did not respond to a request for comment on the filing. Oliveira did not immediately respond to a request for comment.

Brazilian-born Ghosn was arrested last month and indicted this week in Japan for allegedly under-reporting his income. Nissan also accuses him of diverting company funds to pay for personal expenses. He has been held in detention in Japan since his arrest.

Sources familiar with the matter told Reuters that Ghosn’s office had formed an advisory role for Oliveira and compensated her, although an internal investigation by Nissan had found no evidence of any such work being done. The sources requested anonymity because the information is not public.

The Ghosn family representative did not respond to a request for comment.

BATTLE OVER RIO APARTMENT

Nissan is already fighting Ghosn and his daughter Caroline to secure the contents of a beachfront apartment in Rio de Janeiro that Ghosn used when he ran the company, and which it says may contain evidence of wrongdoing. Ghosn and his daughter had sued to retrieve what they said were personal items, such as photographs, jewelry, watches and books.

On Thursday, Nissan faced a setback as an appellate judge ruled that Ghosn or his daughter must be allowed into the apartment during a 24-hour period to retrieve personal belongings. The decision called for two judicial officers to be present, and also allows Nissan representatives to be present.

It was not immediately clear whether Nissan could appeal the decision.

The Rio de Janeiro apartment has become the center of a side battle to the criminal investigation in Japan, which Nissan says was the result of an internal probe into Ghosn’s wrongdoing following a tip from an informant. Nissan says it has found three safes in the flat that could contain evidence of Ghosn’s alleged crimes.

In court papers, lawyers for Ghosn said that since Nissan had entered the apartment after his arrest, the automaker could have “inserted objects or documents that could compromise his good reputation”.