I have no idea if I am even on the right path here, but this was my first impression.
She sounds like a little girl who has become accustomed to not having a "voice" in anything regarding her own circumstances. Because she hasn't had a voice, she has learned to just "go with the flow" and it may not even occur to her that she has a choice whether to like it or not, it just IS. Did that make any sense?
What happens when you try to draw her out with lots of questions? (the kind that require more that a "yes" or "no" -- particularly questions about how she feels about something or how an event made her feel)
What if you watch a movie with her (one that evokes lots of emotion; happy OR sad) and discuss it with her? Does she respond at all?
It is almost like she has totally disassociated?
I don't really think any of that was helpful or answered your question, but you just made me wonder...
Also, does she have any autistic-like behaviors? I know sometimes kids with autism spectrum disorders are totally not in touch with emotions and can have a very difficult time bonding.