Earth Magic by Mallory Loehr

Polly and Sam are off to another adventure.  They find a piece of parchment hidden under a rock that gives them instructions for baby-sitting said rock.  When they bring it home people are surprised to find that the rock can move!  All of the adults are quite blind to the rock moving by anything magical, though.

Remember Joe from the last book?  The characters sure don't!  Reading just this book, it wouldn't even enter your mind that they had an older brother.  I don't think the word "Joe" came up a single time.  Wait, what am I typing about?  Who's Joe?  I don't know what I was thinking....

Instead of Joe, there is Audrey, an annoying but cute six-year-old who follows everybody around.  She sucks her thumb, which is odd for age six, but it is portrayed as normal in the book.  I liked her, but I don't think that just because a new character is added, an old one has to be completely forgotten.