Yes, it is possible that two sisters or two females for that matter would lay eggs (infertile) during nesting season if they do not have a male mate. Twenty eggs seems to indicate there are two females as the largest successful clutch/brood was 13 in England. One of our sites had 11 eggs but only 9 cygnets were successfully hatched. So, this does seem to indicate that both birds are female, however, you would need to conduct sexing or DNA sequencing to be sure. The Regal Swan