My aunt is getting more and more paranoid. She thinks the neighbor is stealing her stuff while the house is locked tight. I need to get her to the doctor. What's the best way?

If you don't have durable power of attorney, patient advocate etc. get it ASAP.
Talk with her doc if she has one & explain the situation. He/She should be able to help.
If she is a danger to herself or others you can call the police & have her taken to the hospital for observation & evaluation. This should be a LAST RESORT!!!!