You need to capture the cygnet and take it to an avian/waterfowl veterinarian that can sex the bird. If these are two females, this would be great because you would not have to worry about cygnets. The swans will nest and they will produce eggs, but obviously, none would be fertile. If you introduce a male, then one of the birds will be displaced. The dominant pair will rule the habitat which means that your original female may be chased from the habitat which is not something you want as this is her home. If the cygnet is a male, there is a possibility that he will stay around and will eventually mate with her. Being birds, this is not like mammals and there should not be a problem with inbreeding.

Finally, if you have swans, most states require that the birds be pinioned (this should occur at 1-3 weeks of age so not to have to use anesthesia which can be hard on an older swan) rendered unable to fly. If state officials find the swans flying, they can and will shoot them so as not to start a feral population. So, it would really be good if the cygnet stays and does not fly elsewhere because there are no guarantees that it will survive. The Regal Swan