The 64- and 55-year-old sisters, who run real estate and finance companies, are accused of violating Japan's inheritance tax law, the Mainichi Shimbun reported Wednesday.

Prosecutors and the Osaka Regional Taxation Bureau reportedly raided 10 locations Tuesday, including the sisters' homes.

Prosecutors allege the two inherited about $59 million from their father but didn't report most of it and so still owe $28.5 million in taxes.

The report said authorities found almost all of the concealed cash in dozens of cardboard boxes and paper bags in the older sister's home. She denied the allegations, saying she had earned the money herself.

If proved, the case could become Japan's largest inheritance tax-evasion case, the Yomiuri Shimbun reported.