She says she has family problems and after she said that, I didn't push her further because I thought I might upset her. As for me, I don't have a father in my life(because he left), I feel like my mother doesn't appreciate me and my little sister was recently diagnosed with diabetes. I can't keep up in school and I have all of the teachers pressuring me and I fear that I might not be something when I get older.
Well, as you couldn't tell much about her situation, I can only go into yours for now (though I'm not a shrink or anything):
Instead of grieving about a bad family you cannot change, make some good friends, such honest friends that you can rely on. I'm sorry for your sister, mine only has a bit of dyslexia, so I can't really give advice with that. About school, I can only tell you that nobody's good at everything, but everybody's good at something – or at least has the potential to become good.
In my opinion, try to think about your talents more than your interests, and figure out what subjects might support you in polishing those talents. Ask a reliable teacher for advice if you don't know what is related (jobs, skills) to the things you feel talented at. Luckily, you're still young and have some years to decide what you want to make out of your life.
When you have no aim, you can only stay passive and eventually drift away into nothingness; I know, because I've been down there for a long time, and had to crawl out almost only by my own strength. So having a goal in life (doing something, possessing something or becoming something) can be a thread you can row your whole on like beads. My depression ended when I found something to live for, and for me it was becoming someone better, though it might be something else in your case. Of course, you don't have to aim for things you can achieve: for me, it was setting a completely abstract idol that I wanted to come as close to as I could. You can think about basically anything, because it's the direction that matters, the direction where you advance in. Of course, the first step is the hardest to take, so be prepared to struggle and keep on striving for your dreams.
Well... I hope this helps. If you have any questions, I'm here.
self mutilation is often a symptom of depression and if it isn't treated, could lead to suicidal thoughts and/or attempts. Your friend needs serious help right now and if I were you, I'd call the cops on her and have her hospitalized and evaluated. She might hate you for it at first, but after getting the help she needs, she will be grateful.
First and foremost, DO NOT cut yourself. You DO NOT want to go down that road.
Second, for helping her, don't make her feel bad ever about it. Google has many ways to overcome it, maybe tell her a few but never push, never stress, and never judge. Be there for her always any time of night or day, and she'll overcome this in her own time.