Unless the seller conducted a DNA Sequencing (most reliable way to determine gender) or probed ( the least reliable method), you may not know gender until nesting season. If the eggs produce cygnets, then you definitely have a male.

Swans will mate with the same gender, i.e., males with males and females with females. The swans will still mate and nest as nature programs to mate and nest, especially during nesting season when the sex hormone levels are high.

Now, having said this, obviously mating and nesting will produce no eggs between 2 male swans. However, female swans will lay eggs regardless of the presence of a male swan. Obviously, the eggs will not be fertile. However, all swans will sit on the nest or eggs regardless of fertility in the same gendered pairings. Eventually, the sex hormone levels return to normal and the swans figure out that they do not need to continue sitting on the nest or the eggs.

If you do have a male from the same family, no worries. Birds unlike mammals, usually do not have the genetic defects from familial breeding. So, this should not be a concern. The Regal Swan