This sound similar to the leucistic (genetic abnomaly) coloration in mute swans. It is nothing to worry about as long as the swans are acting the same as their siblings. If not, then there may be some other problem and you need to get them to a veterinarian. But, if they are eating, bathing, preening and swimming normally, it is probably just a genetic variation of their coloring with no other problems associated. The Regal Swan