There is no such thing as grandparent's rights in CA as regards minor children. She also lacks the right to file a divorce in your husband's name without his knowledge based on a durable power of attorney. You can probably get a restraining order against her but it would be a civil harassment order not a DV protective order like you have against your husband. The forms are available at the superior court in your county office of the clerk.
You should probably consult with a civil or family law attorney. You can get a restraining order based on harassment. You can also prevent the children from seeing the mother in law since she has no rights with regard to them.
You also may have rights to the vehicles if they are community property (that is, purchased during the marriage or with assets acquired or earned during the marriage), and you may still have some financial obligations for the vehicles. If this is the case, you should see someone to help you put a stop to this.