This is an interesting question as geese will commonly accept orphaned goslings, but swans usually will chase and may accidently kill another swan's cygnets should they get into the habitat. Now, having said this, these two parents may be such young parents that they don't have a clue and if the other cygnets were in an area, then they might have allowed the tag-alongs.

Are you sure that these 7 cygnets are not some late hatchings from another nest, same couple? This would also be a little different, but we have heard of a male having two female swan mates at the same time. So, if this is the case, then the male would accept the cygnets and ultimately both females if this is what occurred. Again, good question as we have never seen this in more than 20 years of working with swans. However, anything is possible in nature. The Regal Swan