Hi Nancy
This could very well be the old mate. Swans are known for their fidelity, but there have been reports of a male swan running two nests at the same time with two different mates. This may be nature’s way of ensuring offspring, if there is a problem with reproduction in one of the swans. Although this could be the old mate, there is still a possibility that this is a new mate or even one of the swan’s (now adult) cygnets. Yes, swan parents and siblings can and will mate with each other if another mate is not available. In-breeding between birds is not genetically problematic as it is in mammals. The Regal Swan