Well, us as a species are pretty terrible, we kill hundreds of thousands of animals, destroy habitats, cause extinctions, polite the earth, claim everything as ours, have wars, etc.
/rant
This sounds like a pretty serious issue, if she is legally their child, why don't you talk to her about her legal options and/or just talk to her parents, or better yet, have her talk to them herself about how she really wants to see her brother, as that's the least they can do, even if they don't let her move in. If she needs a place to live, ask your own parents if she can stay at you're house.
Not sure about your specific situation, just some ideas.