Have you tried adding a second fridge, or replacing the fridge entirely? Has the fridge been cleaned out ever? Do they still suffer if they eat from an alternate meal source placed in the home (if you have one), like University's barista bar or any of the store items (ice cream, pizza oven, etc)? Can the mom cook a meal and does it have the same effect if it doesn't go in the fridge? What about if the meal is raw (salad, parfait, sushi)? I wonder if something in the cooking process is 'tainting' the food (either the fridge marks the produce as bad, or perhaps the stove does).
Does the mom have simmunity or the slob trait? If the game thinks the food is spoiled and she's got either of those she wouldn't be affected by it.
One quick fix to try in a non-dynasty game is to move the household to another home via Real Estate then move them back in. Sometimes just that will fix things.
The other quickie is to move the household out to an empty lot, save the household to the bin, save a copy of the now empty home to the bin, delete both household and home, and place bin copies back down. If there's sim-made art and such in the home it's best to put all of that into a sim's personal inventory (or the family inventory in a pinch). If all goes well the family keeps all relationships and careers and the house thinks it's brand-new again. (As always, save before trying either just in case)
PS - Sorry for so many questions. I find this bug you're encountering very unusual and most curious. It's quite the mystery!
