Caring for someone that cannot accept their reality of no longer being independent. What do I do?

I care take an 85 year old, never married woman who had a stroke 8 months ago. She has recovered as much as she will, according to the doctors. She believes that she can drive, and will again. Also she is always telling me that she can bath herself etc when the aides say otherwise. The problem is and advice please: She has taken the car out on the lawn, then not known how to get the car in park, she can't bath herself without someone assisting her and hovering, and if they don't assist her she will fall - so they won't let her do it herself. I have spoken to her family about the car and other things, and they say, " just let her believe these things and don't take away her hope…" Taking the car out is dangerous, not only to her but to others. She has vascular dementia, when she gets agitated she acts out in strange ways, including going for the car. I am seeing that her life long independence is now a hinderance. Any suggestions to get someone like this to trust? or listen?