How should I respond to paranoia?

My diabetic 93-year-old mentally sharp mom, recently accused my wife of stealing or going through items in her condo while I am out running errands with her. This is the second time in the past three months that she has made such accusations. She doesn't seem to budge her conviction that family members are out to take from her, even though we have no keys or any means to get in to her condo without breaking down the door!