It depends on the facts surrounding her statement to the police, if there were witnesses, and what actually transpired. There is no guarantee that the DA won't file. They are used to people changing their minds once the police become involved. This is called "battered women's syndrome" and victims often become recalcitrant in not wanting to tell the story they initially told the police. You should hire an attorney who will go with you to law enforcement and speak with the detective assigned to the matter if there was some kind of mistake. This will often prevent them from filing on you (or your sister in this case). Once a case is filed and is not a DA reject, it is really difficult to "undo". You have to fight it like any other case and DV carries some heavy fines, penalties and sometimes jail. Usually if the DA files the case and you don't go to trial and win, you will be sentenced to DV classes in addition to fines, probation and sometimes restitution, which is expensive and time consuming.
Filing a false police report usually is charged as a misdemeanor violation of CA Penal Code ("PC") Section 148.5, punishable by imprisonment in the county jail for up to one year and a fine up to $1,000. However, there are exceptions: Knowingly giving a false name to police or using the identity of someone else on any police report, other official or financial report can be charged as a Felony under Penal Code Section 529 PC. Also, if the police report is made or required for filing an insurance claim, knowingly providing false information, including a false report of a theft or loss, false report of damage or false value of insured property, it is considered insurance fraud. The false report mostly likely will be prosecuted as Felony insurance fraud under Penal Code Section 550.
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