Your options include:
First and foremost, check on your insurance coverage with your homeowners' insurance.
Have someone watch her.
Obtain a restraining order.
The thing is, you can't prosecute someone for thoughts.
It would restraing her from destroying or stealing your property. It would be an added way to punish her later, if she violated it.
However, now that I understand more clearly what she threatened, since it's a definite threat you can file a police (or sheriff) report stating that she has threatened you. If the other people in the house cooperate with statements, that will help.
Still, however, the best thing to do is to keep an eye on her. And try to find her a new place to live.
One other option - work with your local mental health folks. If this woman is making threats, it's possible that they will pick her up and take her off your hands for a few days for observation.
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