It was in my language arts text book. As far as I can recall, it was about a family that was new to town. The protagonist befriends a little boy in the family but the boy can never come out when it rains. It is later revealed that the entire family is made of sugar (or maybe it was salt...some kind of dissolvable, powdery substance familiar to grade-schoolers).
The illustration in the edition of the book I read was an Alex Katz painting of a woman wearing a headscarf and holding an umbrella.
Maybe someone else has read the same book?