Sisters and Brothers was a take on the animal world that is not normally explored in literature, and it's a shame that this angle hasn't been looked at more closely before. Kids generally like stories about animals, and kids who have siblings, as my niece and nephew do, may find extra enjoyment learning about the sibling relationships among animals.
What was especially interesting about Sisters and Brothers was the diversity of the animals covered. Myna birds, anteaters, shrews, and grebes (among others) were all represented. The book also served to reinforce *my* younger sibling's notion that older siblings are always bossing and tormenting the younger. Tsk. She's such a drama queen. :-)