You answered your own question in the last paragraph of that letter. Her step dad has hit her. You know this and yet you ask me how to get social workers crawling all over that house? Um... HELLO?!
But ok, barring that (since, yeah, it is hard to prove, I would imagine), there is one law that could help her. She should file for emancipation. States don't often grant emancipation and she has to be a certain age to even apply (varies by state). At the very least, it will have Child Services taking a very hard look into her home life to find out just what is so fucking bad that she wants so desperately to get away from.
You should also understand that the number one goal of any social worker is reunification. Even if she's taken away and placed in a foster home for a while (where you probably won't see her), the primary goal of the state is to get her parents counselling and whatever else they need to become the parents they *should* be, so that the state can give her back to them. And even that is further along in the process than most people think. Social workers don't just come in, see a bad situation, and take a kid away from their family. Oh no, my friend, there's a long process involving recommendations from experts, a full investigation, etc. Basically, sorry to disappoint you, but there is no "quick fix" to this. There's no hotline you can call that guarantees getting a kid removed from her home. That shit only happens on TV.
I hate having to say this in any DG letter, but the honest truth is that there is virtually nothing that can be done. Yes, starting a CPS investigation will prevent them from sending her out of the country (unless they'd like to go to fucking jail, lol), but it's likely to end as a fruitless endeavor that only delayed the inevitable. Run away with her for a day or two (it can't get any worse now, right?), spend some time alone together, and say your good-byes. Sorry bro. I really am. Let her know that, if they're gonna fuck up her life anyway, it's time for her to stop babyfacing their stupid asses. She needs to become a royal bitch and tell them to go fuck themselves, right fucking now. And when she turns 18, cut off all contact immediately. Parents can be real fucktards and hers sound like they don't deserve the GIFT they've been given by having a daughter. Fuck 'em.
With Seething Anger,