You have been provided misinformation. Swans, in fact birds and some mammals will breed with their own siblings. In swans, you don't have to worry so much about genetic anomalies. In fact, we have only seen a color variation which is a leucistic gene found in many swans, Mute swans especially, and does not cause any other problems.

As far as the introduction, did you introduce the two birds by penning the new juvenile? By placing the juvenile bird in an enclosed pen (top to bottom) with a feeder inside, zero entry ramp, 1/2 on land and 1/2 in water, the juvenile can get acclimated to the new habitat, the older swan (and she to him), your feeding system and any thing else he will learn by staying in the pen for approximately 1-2 weeks. If you see any aggression after this time, then you probably need to find another home for the young juvenile and let the female stay alone. This will not hurt her and it is an individual attribute whether she will take on another mate. Usually, if proper introductions are made, the females will accept another male. If in your case, things have started to settle down, then you may have a chance that this will work out. Just keep a close watch on him so that he does not get too beat up or he leaves the pond to get attacked by predators. The Regal Swan