Complex sentence: In according to legend, Cassandra had instructed her twin brother Helenus in the power of prophecy for him to be a prophet and like her Helenus was always correct whenever he had made his predictions, but unlike his sister people believed him.
Split into simpler sentences: In according to legend, Cassandra had instructed her twin brother Helenus in the power of prophecy for him to be a prophet. Like her, Helenus was always correct whenever he had made his predictions; but unlike his sister, people believed him.