Rules by Cynthia Lord is a work of fiction. To be more precise, it is a work of realistic fiction for young adults. This means that it deals with themes that one is likely to encounter in the real world, but in such a way as to make the book appropriate for young adult readers.
Although the book is aimed at a wide audience, it will doubtless find especial favor among those like the 12 year old narrator Catherine, a non-disabled child with a disabled sibling. The book presents, in unerring detail, the stresses, strains, and challenges that growing up with a disabled family member can often present.
The focus of the story is very much on its protagonist, Catherine. This has invited criticism from those who feel that Catherine's disabled brother David isn't adequately humanized in any way. It is Catherine's experiences that lie at the heart of the story, not David's. Critics argue that his sole purpose in the story is to act as a plot device to show us how difficult it can be for non-disabled people to cope with disabled family members.
Having said that, even critics of the book have acknowledged that the book gives a generally realistic account of what it's like to live with someone who has a serious disability. In that sense, one can say that Rules, whatever shortcomings it may have, does at least show all the signs of conforming to the genre of realistic fiction.