How cassandra got pregnant from John Denver since he was sterile?

The scenario you mention is likely fictional or a misunderstanding, as John Denver was known to have had a medical condition that affected his fertility. If Cassandra got pregnant from him, it could be explained through artificial insemination or other reproductive technologies, which are not limited by a partner's sterility. Alternatively, it could also be a plot device in a story where the typical rules of Biology don't apply.