realistic fiction book between 100-250 pages

theres a boy with an older sister. one day, he finds cards numbered 1 or something like that. he puts them in a box. he goes around trying to collect more. the cover is a girl with a blue bob with like flowers on her head. the background is in beige. in the process of collecting the cards, he cant seem to find one, he finds the girl and she gives him the last copy of it.