Okay, so my dilemma is this. I'll simplify as much as I can but could really use advice.

My WIP is a YA fantasy. (Contemporary fantasy? Urban fantasy? I don't know.) Healing is one of the powers people can have in this world, but all the powers are somewhat limited. One girl (a secondary MC) has considerably more power than others. It turns out that the headmistress of her school is sick. (She has to be seriously ill, it doesn't necessarily have to be cancer.) This girl is healing her on the reg, but she can only keep it at bay, not get rid of it.

Then later, it turns out another character's mother has cancer (or the same thing the headmistress ends up having) and that causes some problems/moral dilemmas when he finds out this girl can heal at that level.

SO the problem I have is: I am worried about using a serious disease (that has been handled very thoughtfully by other people) as a throwaway plot device, but the people who have it are not the main characters. One does not appear in the book and the other is the villain. This isn't really a story that can address the struggles of being ill, though the character whose mother gets sick will deal with some like "how far would you go to help your family" stuff.

Do you have problems with the situation as described above? Do you think it's insensitive or should be tweaked in any way? For the story to work the headmistress has to be sick, somehow. (I have also thought about it being a thing where she's really old, and is using this girl's magic to keep her body from breaking down).

Any help you could offer would be greatly appreciated.

Sorry for being longwinded, and thanks.