Guardianship

What makes guardianship legal? I have a 17 year old that is having very difficult times at home with her mother and step-father, (and has been for the last 4 years that I am aware of) and I have been watching her go downhill a little more each year. She is very bright and intellegent and doesn't understand why her mom just continues to throw her away. She loves her mother but can't stand being in the same house with her because she is constantly being blamed for their financial status, their fights, constantly being cut down to the core, (being cursed at daily). She is at her wits end and doesn't know what to do. There is a lot going on that is too lengthy to put in writting but to some it up last year her mother punched her in the face and she came to my house for help after her step father threw her into a door that same evening. I told her in order for her to stay she needed to call the police to report what happened so I didn't get accused of kidnapping her. She did and explained everything to the police where in return he went to her home discussed what happened with her parent and in the long run was arrested for abuse, (the mother that is). But refused to allow her to stay with me and made her up root her entire life and move in with her grandmother in a different state so she had to change schools for the time being and all. Now she is back home again and back in her normal envirnment but nothing has changed if anything it has gotten worse. Now that she is back home her mother has done nothing but ridicule her and blame her for everything in their life going bad and continues to throw the jail thing in her face. I have had a couple of conversations with her mother and told her this is a very difficult age for any girl and for her to think about when she was 17 and then she would calm down a bit and talk to me and I would give her the best advice I know with having 2 teens of my own, (and I thank God everyday that my boys are very well mannored and behaved) but she would turn very nice talking to me and then get all over her daughter about stuff again. Well now it has come down to her mother telling her to pack her bags and to get her butt over to my house and to have me draw up papers for guardianship and she will sign them. I have no clue what to do and am seeking some much needed advice. If anyone has a clue what I can do please respond it would be greatly appreciated! By the way my husband and myself are more than happy to have her live with us when she is here which is most of the time she is nothing but respectful to us and very well behaved. She is just very depressed at this time and doesn't understand why her mother doesn't want to love her and we do alot of crying together and a lot of hugging and telling her she is loved very much by us, and if I had a daugher I would want her to be just like her. She has so much compassion and so much love to give and I don't want to see all of her wonderful characteristics destroyed. Thank you all for letting me vent my problem to you and I am looking forward to hearing any advice I can get!