Hello. You did not enter the text to which this question refers, which makes it impossible for it to be answered accurately. However, I will try to help you in the best possible way.
If Ellie prefers one of her brothers, it is because that brother treats her well and she identifies with him. In this case, we can say that Ellie and Billy's personalities are similar and match each other. This makes her feel a strong sympathy for Billy that she does not feel for other brothers.
