A couple of ideas:
This one is a journal that the Big Sister gets to write (with stickers and fill-in-the-blank sections) about the first year of their new sibling’s life. If she’s a crafty sort or likes to read, this might appeal.
Perhaps you could add to it a book like Just Mom & Me or Just Dad & Me – by the American Girl Doll publishers (they make great girl-centric books, really)..and this can be a special thing to do together w/ mom or dad to remind her it’s not all about the new baby.
And finally something like the Girls Guide to Everything might be cool…and you can tell her she can teach the new baby in time.
Lastly..if she’s into art or something..maybe some coloring books or a nice journal or pad of paper with a set of colored pencils would make her happy (and help keep her occupied when mom & dad are busy caring for a newborn) as well.