J.C.
It sounds like she seriously needs some love and attention. If there are that many people in the house... she might be feeling like she doesn't matter and so what does it matter what she does. I know that when I was a teenager I felt completely abandoned because I wasn't raised by my parents. I did some things that I wish I could take back, but now I look back and it's part of what made me who I am. The difference is that I had someone stand in for my family who I figured didn't care about me. You're busy with your dad and your boyfriend. Her father left her behind and now has a new family who he loves more than her. She is hurting inside and has no one to turn to. So she is going to take it out on herself. If she hurts you...she might feel badly, but in the long run she doesn't care because she's only focused on her pain. At that age teens have a hard time "feeling" someone else's pain.
Give it some thought to scheduling a certain night to be with her and do NOT break it. Also, get her involved with Big Sisters. She needs people to talk to!