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.
“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass