You will probably get the car back, but not until officials are done building and then prosecuting their case.
In no way am I offering you legal advice, and in no way has my comment created an attorney-client relationship. You are not to rely upon my note above in any way, but insted need to sit down with counsel and share all relevant facts before receiving fully-informed legal advice. If you want to be completely sure of your rights, you must sit down with an experienced criminal defense attorney to be fully aware of your rights.
Your mother should consult with an attorney who does defense work for personal injury cases. They will know what to do to try to shield her assets from being exposed to any potential lawsuits.
It is possible that the car will eventually be returned. Your mother can speak to your brother's criminal lawyer or hire one for herself to speak to the prosecutor and the police. She should check her insurance to see if there is enough coverage to cover the potential liability for the possible lawsuit. There is a danger that an award above the amount of coverage could endanger her other assets. Good luck.
This answer is only for informational purposes and is not meant as legal advice.
From your post I take it this is a NYC arrest. The decision of whether to seek civil forfeiture is up to the NYPD; not the district attorney. In my experience in NYC third-party owners almost always get the cars back. Even most defendants will on a first (and sometimes even second) offense. Assuming the police are not seeking forfeiture you may be able to get the car back before the criminal case is over. The DA will usually enter into a stipulation with the driver stating that there was nothing mechanically wrong with the vehicle before issuing a DA's release. The police will not give back the car without either a DA's release or a final certificate of disposition.
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