Cassandra of Troy was Helen's sister. Not only is she a mythological figure, tied to the gods, but she is shrouded in tragedy too.
It is said that Apollo wanted to seduce her and therefore gave her the gift of being able to foresee the future... Cassandra didn't give into his advances and because a godly gift cannot be taken back, Apollo cursed her with the fact that no one would believe her.
Setting the scene for what was to follow.