Missing May by Cynthia Rant is the story of a young girl who loses her parents when she is very young; as a result, Summer is passed around among her relatives and grows up mostly unnoticed and feeling unwanted.
At a family reunion, some elderly family members, May ond Ob, take Summer and become the loving parents Summer needs. May is everything Summer needs in a mother.
[May] understood people and she let them be whatever way they needed to be. She had faith in every single person she ever met, and this never failed her, for nobody ever disappointed May. Seems people knew she saw the very best of them, and they'd turn that side to her to give her a better look.
For six years all is well, but then May dies and Ob does not seem to want to go on living. Summer is afraid that everything she has known as home and family will soon be gone, but, with a little help, Summer and Ob manage to overcome their grief.
The genre of this Newberry Medal award-willing book is juvenile fiction. Goodreads categorizes it as children's fiction, young adult fiction, and realistic fiction; Barnes and Noble list it as children's fiction and has reviews from several children's fiction magazines. The author's website lists it as a "middle-grade book," which is probably the definitive word on the subject.