Does your mother-in-law have a permanent address and does she have current home or tenant insurance? If so, her policy will extend to her temporary address at your home. However,she would want to verify with her carrier. There might be a limit, such as just 10 percent property coverage on belongings when they’re away from the person’s main home.
What do you mean by temporarily living with you? Is this a short term plan, or is her address now your address? If she is now a household member meaning she has changed her address, she would be considered a resident insured and your homeowner’s policy would provide coverage to her valuables.