it would be hilarious if this one pulls the "incest bad" thing tho lmao (I've only seen one do that)
Nah Incest would leave chronic issues from birth, the fact that they are sick for the past year, and the author has set the stage with the awful tasting food, leads me to believe that it's a nutrition issue.
AKA the commoners don't have any other recourse than eating said foul tasting but nutritious food, but the nobles and royalty choose to eat some kind of tasty but lacking in nutrients dishes.
Or perhaps the nobles are eating something that in the long run binds certain vitamins and minerals, like how raw egg yolk binds B7 making it insoluble in your body and thus un-absorbable.
Finally there is a chance for the commoner's food to be intentionally made "unpalatable" by some third party ,just so they can sell normal tasting, yet poisoned food to the nobility.