It's kind of silly to include a subplot where the little sister gets an incurable disease that has a 100% mortality rate when your own cover shows a picture of the two siblings in the future meaning that she'll obviously be cured.
Not to mention that it's nearly impossible to build tension in these types of stories anyways because of how it's done, Even in the off chance that the little sister dies somehow the main character would just gain an ability later on that would allow her to revive, it's so common in this genre that it's kind of stupid at this point.