Not a fairytale

My niece lives with me. She is 14 and her parents were killed in a car crash 4 years ago. I was happy at the time to take her in and raise her as my own but I must admit that it has been a lot more difficult than I thought it would be. She is now having a lot of behavior issues at school. She is hanging with a very bad crowd and she is experimenting with smoking and drugs and drinking. She was caught shoplifting before Christmas and the police brought her home a month before that suspecting her of an act of vandalism that they couldn’t prove she committed. I have 2 kids of my own aged 8 and 11. I am worried that she is a bad influence for them. Because she is older I have higher expectations for my niece than for my children but she interprets that as a double standard, as if she is Cinderella and we are the mean family that makes her work while we play. She thinks we treat her differently than we do the other kids but it really just isn’t true. I am not sure what to do with her and I am thinking about some type of boarding school for kids with conduct problems. If she was my own kid I think this would be an easy decision to make. She needs an intervention and she needs it now and the path she is on right now in this home and at this school with these friends she has is not a good one at all. But since she already feels on some level that she isn’t a real part of our family I am worried that she will feel like we are sending her away because we don’t want her anymore and that instead of making things better, this is going to cause her to get worse. She has had a really hard time and I want her to succeed in life. I also really do love her like one of my own. How do I get her out of this bad situation she is in without making her feel like we are pushing her away?