Legal Question in Real Estate Law in California

Well you are asking on a legal site. The only option the law provides is for you to sue her for the money. Once a lawsuit is filed you would also be able to ask the court for an immediate order that her bank account be frozen so she could not spend the money. There may be other options outside the court/legal system, such as getting friends or other family to try to get her to cooperate, but that is outside the scope of LawGuru.

More facts would be needed in order to give you a better answer than "sue her." How, for example, did a check for $250,000 payable to you get put into your sister's bank account? Accident? Some intentional arrangement? Fraud or theft? When did you first learn that your money was in her account? When, if ever, have you asked for its return? Was your request refused, and if so, on what basis? Among the reasons why this additional information is so important is that there may be a "statute of limitations" problem on a suit demanding return of the money after ten years.