Good news that the dad is watching over them. However, if the mother died of neurological problems, it could mean botulism, aspergilliosis (fungal infection) or even encephalitis, swelling of the brain basically. If the neurological condition was caused by botulism, then the other swans are susceptible if they eat something or in the same area because this is a form of food poisoning. In the case of the fungal or other infections, these may spread or can be infectious to the other swans. Just keep a close eye out for any abnormal behavior. You may not be able to save the swans if they are afflicted by the same problem, but it will give you a heads up to try and save the others if you can diagnose the problem early. Hopefully, this was just an anomaly+ and the other swans will do just fine. Good luck. The Regal Swan