Somehow, when they were all young adults, a curse was placed which would kill Eva. Blue Fairy stepped in and provided a protection spell, a magical cuff, but it would only protect Eva until a new royal was crowned. When the new Princes (Snow) came of age and was crowned, Eva would die.
That part confuses me because Cora says “poison looks good on you” so I don’t think it was meant to be a mysterious ailment and Cora just took advantage. She set that whole thing up, just as she set up Snow’s horse running away the day Regina saved her.
When Eva takes the crown from Joanna, and gives it to Snow, watch how Eva is stricken. An odd little barely noticed glimpse at her wrist which Joanna and Snow have just touched. It’s very very slight, but I notice the glance at her wrist every time I w tech that clip. It’s possibly just my imagination