This is a wonderful idea.
The Saggy Baggy Elephant was my favorite.
My kids loved Parts, but I don't remember the author. It's about a little boy who thinks his body is falling apart when he finds thinks like lint in his bellybutton, earwax, etc. It's really cute.

Oh, and, Where the Wild Things Are is an excellent book, too.

A must for your sister is I love You Forever.