You could be held liable for any damages that she causes. When you were appointed guardian your "ward" was declared legally incompetent by a Court of law. Therefore you should have the ward evaluated to determine whether she is competent and if a medical doctor determines the ward is able to manage her own affairs petition the court to terminate the guardianship.
If the professional determines she is not capable of managing her own affairs, then prevent the ward from driving.
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I agree with Mr. Brockett and would like to add that you should also speak to your automobile insurance agent about coverage. Your agent may be able to tell you what they need for documentation regarding your ward's competency or if she's insurable at all.
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