We would suggest you capture the cygnet and return it to the parents and his siblings. At 13.5 weeks of age, the parents have sufficiently bonded and should take him back. Just watch for any aggressive behavior which may require quick intervention on your part.
You do not want him to go near the other family as the adult birds will not accept him and will beat or chase. This will cause the cygnet stress possibly resulting in injury, dehydration, or starvation. The Regal Swan