Rafi and Rosi: Carnival! by Lulu Delacre is a touching story about brothers and sisters. Rafi the frog is willing to work exceptionally hard to make his younger sister Rosi happy. After he made a mask to prepare for the Carnival in Ponce, he also made her a wagon-turned-float to take her up and down their street in her own parade. I love this book because of the surprises at the end: A bit of history of the Carnival in Ponce, Puerto Rico, instructions on how to make a wagon float, how to make a periscope, and how to make a vejigante mask.
Another cute book about brothers and sisters worth checking out is by Laura Numeroff, What Sisters Do Best; What Brothers Do Best.