The mouse story is a little scarey. But I predict that very soon, her lonely single mouse will be joined by many other mice and she will realize that she screwed up another house. You should tell her, as she is your beloved, if looney sister, that mice carry HANTAVIRUS pulmonary disease. Contact with rodent urine or feces, evn small mouse feces, in 1-5 weeks incubation can start with fatigue, muscle aches, headaches, dizziness, chills, nausea. Then the REAL fun begins as coughing and shortness of breath happen as your lungs fill with fluid.
HANTAVIRUS is fatal 38% of the time.
Presumably she has not house trained her mouse. No, it doesnt count if she put a barbie house toilet under the dresser. Mice are notorious for not washing their hands before returning to work.
Or she might make the news with a case of bubonic plague, or maybe one of the other diseases that are well known in the rodent population.