Normally, the eggs will hatch every day or every other day after the first egg hatches. If the parents are still sitting on the nest, then they know something that we don't.

If the parents abandon the nest and go about their business of raising the first cygnet, then it means that the other eggs were not viable. There is also a chance that if the mother leaves the nest with the first hatched, if anything else hatches after that, you will need to find a poultry or waterfowl sanctuary

Nature will not wait for the last born, runt to keep up and so trying to introduce the young bird to the mother once she has abandoned the nest is not going to work out well for the last hatching. Hopefully, she will leave and nothing else will hatch if further hatchings do not occur within the next few days. The Regal Swan