Just a thought,
I'm not convinced the copepods ate "viable" eggs, though I could be wrong.
Were they white or a creamy dull yellow. Pure white eggs are often duds, and creamy colored or yellow eggs usually are viable.
If this was the first spawn for the Parachromis, they may not have gotten it right, it often takes practice.
So the eggs could have been infertile from the get go, and infertile eggs degrade quickly.
Experienced Parachromis parents constantly mouth viable eggs, cleaning and ridding the eggs of debris, and parasites.
If the eggs were not viable, the parents would eventually just ignore them.
Also, if your sister was tossing in food by the train load, this alone could overwhelm the eggs and smother them with debris, which causes them to quickly fungus, even if temporarily viable. Eggs must have a free flow of oxygen to survive.
The copepods tend to swoop down when fungus is abundant, making it appear they are the bad guys, but they may have just been cleaning up.
the eggs above are infertile, stark white, no chanceoof getting fry.
the eggs in the pic above are fertile and produced many fry.
below is another example of fertile eggs
and another
as eggs mature just before and after hatching will also get white, and is when parents need to remove the fry to another area, as fungus takes over the remaining egg "shells".
There will always be a few infertile eggs in any spawn, and they are obvious, because they are while.
A good parent will get rid of the infertile ones, you can easily see them below.