Call or write your mother in law and have your husband do so as well and tell her she needs to give you everything within 24 hours. Tell her you don't want to do so, but if she continues to refuse, you will call the police and ask them to escort you to get everything. Be ready to show proof of ownership with the title and registration and proof of insurance for the car.
If the police will not assist then you need to prepare and file an ex parte application, declaration for you and husband and a notice of lodgment with the paperwork or exhibits proving ownership and get a court order for mother in law to release the property. Whatever you do, do not get into it personally with your mother in law and get yourself arrested or damage the relationship beyond repair. As bad as it may seem now, she is still your husband's mother.
Legal disclaimer:This message does not constitute legal advice and does not create an attorney-client relationship. Any statements are made for general informational purposes and do not constitute legal advice. No attorney-client privilege is created by this communication. Attorney is licensed in California only.