The swans are getting older and stretching their wings and their exploring habitat. Even though mom is still with them at this time, eventually the majority of the family will eventually leave to find mates. They may or may not return based upon how the family unit (especially mom) allows them to stay in the area. If one of the cygnets is a dominant male, he may even return to become mom's new mate. Yes, this happens in swans and other wildlife. In swans and other birds, there is no negative genetic problems.

Your feeding is allowing them to have an alternative (supplemental) food source so that they can explore for longer periods. In all probability, there is a nearby suitable habitat (pond/water source) that is allowing them to stay for longer periods of time. They usually do not fly from local pond to pond at night.

Once the migratory period begins, there is a possibility you will not see any of the family again until spring time, especially if your area sustains severe inclement weather and the water sources freeze. The Regal Swan