Are You There, Baby Bear? by Catherine Walters
So, this one has a twin boy and girl (I know not exactly what you are looking for), but is a very sweet story. It is more a story geared toward a child awaiting a sibling or twin siblings, as the case may be. I bought it for my oldest when she was waiting to become a big sister.

And have read it numerous times over the last 7 years. Still not sick of it. That says a lot.
