Not sure if you'll see this but you were definitely on the right track with why cassandra hates eugene. I think it's also partially the fact, if not fully because, he just got everything he ever wanted without any effort. Cassandra has been having to fight tooth and nail for everything. To get the princess duty, to get recognized by her father, trusted by her father. Not seen as a liability but as a strong member of the team. Eugene basically became prince overnight