@mom2collegekids The description of your sister’s situation is exactly why some markets - San Francisco and surrounding area - have housing units sitting empty. Owners do not want to risk this type of scenario.
I know of a handful of homes currently empty. They are managed by the kids of elderly parents who are now in assisted living or other type of arranged housing. While it would be nice to rent out the parents home, those that do not absolutely need the income will keep them vacant. Why. Because if either the parent or the child needed that home - to live in or to sell - removing the tenant is a nightmare.