How to treat a teen guest

[Viki63]

Just looking for some thoughts:
I have a 15 year old girl at home (doing very well in school), & an 18 year old son who lives at his friend's house, couldn't follow our rules so he left.
Meanwhile, son's girlfriend who is now 17 was thrown out of her house by her mother, to protect her from an abusive older (adult) brother who rules the roost. We've allowed her to stay with us for the last several weeks, and helped her get into school and counseling.
She resents us because she feels like she "has" to stay with us, doesn't speak unless spoken to, spends every weekend with her mother (when brother is away) & only spends weeknights with us as a place to stay for safety. Her mother may have some developmental delay issues, not quite sure why she can't step up and take care of her child but this girl has really fallen through the cracks, had been out of school for several years & allowed to run wild.
She also spends some nights with girlfriends, but won't tell us their names or addresses, says it's ok with her mother. Our house rule is we have to know where & with who she spends the night, or I call the police. I spoke to her mother the last time & she said not to call the police, she believed she was with a girlfriend..But I told the girl that the next time she's AWOL I will report her as a runaway.
I am concerned for her safety if I ask her to leave, but worried for the example for my daughter if she stays. She is working with a group advocating for homeless kids, who plan to help her get back with her family in a few months, but I'm not allowed to talk with her counselor due to the HIPPA laws. (The girl won't give her permission.)
Some of my friends say push her out, but she says her mother won't allow her to come home since the brother was abusive, & she has nowhere else to go but sleep in the parks.
My husband is very generous and willing to go with whatever I decide.The girl is really not a problem as she never speaks, just sits at the computer on Facebook for hours. Though it is uncomfortable to have this silent presence in the house who looks like she's angry, but she is rarely here.
(This situation reminds me of the movie Precious, if you've seen it you know the teenager was very rude & dysfunctional, because she was treated like dirt so she knew no better. I know this girl won't change overnight, but am hoping that some of our example of courtesy will stick with her, and at least we've gotten her into school & some counseling.)
I am thinking of telling her that unless she allows me to talk with her counselor/advocate, she will have to move out. I haven't yet because possibly this is one area where she feels like she has some control.
I want to be Christ-like for this girl, but it's hard to set a good example when I rarely speak with her, and it's usually to try to set limits. (This is the same "Buddhist teen" I posted about earlier)
I'd appreciate an objective eye on the situation.