I think you’ve already gotten some really good advice! Also, it could have a little of the dynamics of the siblings in “Series of Unfortunate Events” in that the older ones are very intent on protecting the younger, innocent one.
Also it makes me think of the Pevensie siblings- Lucy is finding her way and her faith, and her older siblings have to gradually discover she is someone to be respected.
I like the idea of it going to her head. What if she felt really overlooked, and being “Chosen” suddenly gave her a reason to be special? Because before that she never felt that way.