There is always the possibility that Spencer is a removed brother of Bella or a sibling of her mate (which may be one of the two birds that arrived with him to your lake). Swans tend to go back to familiar areas.

As far as genetic anamolies through inbreeding, there is always a possibility later down the line. But, swan siblings will pair off, mate, nest and produce cygnets. Unlike, mammals, in-breeding in the swans is not usually detrimental unless there is an immediate underlying genetic problem which will be readily seen during the first or second mating. But, again, usually if anything does appear, it will be years down the road. The Regal Swan