Book for new baby in a complicated situation ...?

Hi. So I'm in a slightly complicated situation. My fiance has 2 children from a previous relationship (two girls, age 5 and 10). My fiance and I just found out we are expecting in January, 2012! Since we've told the girls the news, the younger one has had quite a few temper tantrums which I'll bet are related the the news. I am looking to get her a book or maybe a movie or something that might help her prepare for this huge change thats about to happen, however, all the books say "mommy and daddy", and I'm only her stepmother, so that kind of book wouldn't be helpful. Anyone out there know of a book that may be helpful? Thank you!