While my nephew wasn't interested in this book at all, my niece liked it to a degree. She certainly identified with the older sister, but also seemed to think the sister needed to cut the baby a little slack. She said to the sister, "Well, of *course* the baby can't do all those things that you can! He's a BABY!" I'm sure that her younger brother was thinking, "Ah, the irony."
This is probably a story moms and dads will get a kick out of, especially if they have multiple children. They will certainly be familiar with that older sibling voice, for on that score the author does an excellent job of capturing the authority (or some might say know-it-all-ness) with which we older siblings are wont to speak.