What should I do as far as punishment/discipline for my 12 year old kicked out of school?

My step daughter is now coming to live with us because her mother says she can't handle her. The other day she was just kicked out of the second school this year for FIGHTING! Her mother says the school is sending all her work for the remainder of the school year so she will not be attending school for the remaining month left of school. my question is what should my husband and I do as far as discipline for this. She needs structure and guidance. I was thinking to have her go to library every morning M-F for 5 hours a day. She would have all her books and a lunch with her. Getting dropped off when I take my kindergartener to school and getting picked up when I pick the little one up from school. Is it ok to let a 12 year old stay in the library alone. If this is too harsh could you guys give me any suggestions as to what to do.She has told us in the past that she has a really big probem with her mother....she thinks her mother doesn't care about her and she is always putting other people before her kids...she has also said in the past that she hates her sister 13 (my other stepdaughter whom the 12yr has said has said she will kill herself before. .so I know this is a big reason why she has been acting out the way she has. They are unsuppervised at their mom house cause she is never home. This being kicked out 2wice THIS year but a year ago she was kicked out of another school for FIGHTING....so that makes 3 times kicked out for the same reason. I want her to feel loved, understood, cared for etc but at the same time I want her to know that certain things will not be tolerated as well. thx