Because Melida is powerless, she does not have legitimacy as an heir. Elli then has to be the one to take all of the training and the attention, which was nothing but suffocating for her. And since Melida is practically isolated from Elli, Elli is not able to meet sister and get spoiled/loved by her.
If we're to take into account what Elli said, "You were the big sister I looked up to", we could say that Elli was also afraid of being disappointed. It's like seeing your favorite idol fall down so low.