because a hen only lays 1 egg a day the egg if fertilized will stay in a sort-of suspended animation. This way the hen can lay enough to make it worth setting and hatching them. So the egg stays just an egg as the days go by and she lays more.
Lets say 10 eggs is the magic number for this particular hen. On the day she lays her 10th egg (this could take 2 weeks to lay them all) she then decides to sit on them. Once she has been on them for about 12-24 hours and the eggs have reached 100 degrees and have stayed there, then and only then will the cells begin dividing and an embryo start. This way all of the eggs are on the same schedule and all hatch with in 48 hours of each other. If they did not she would either have to leave the nest to feed the first few to survive or the first would starve as she continued to wait for the later eggs to hatch.
I hope this makes sense. It is natures way of taking care of reproduction and having a brood of chicks leaves the best chance of some surviving and passing down the genes.
Soooooo a fertile egg is really no different than a non fertile one, except given the right temperature for an extended period of time will cause the fertile egg to develop.
Now comes the issue of having silkies for egg eating, they are great pets and great broody mommas but layers they are not. Maybe 100-120 eggs a year and they are very small.
I would try to convince your parents to get a pair of hens that are large fowl and regular layers, or it may take weeks for one of them to be able to make an omelette.