I started cutting at 12 I'm 19 now and I wasn't diagnosed with depression untill i was 17 major depressive disorder until I was 18 anxiety and 18 and manic depressive bipolar disorder until the end of June this year. Talk to your parents have her lithium levels tested and find a therapist. And make sure she likes her therapist because it won't do any good to have one you don't trust and can't open up to.
Be a listening ear for her. That's the best advice I can give. If she's getting help, that's all you can do. The worst thing to do would be to be, "You're quiet today. Are you depressed/thinking about cutting/whatever tody?" If she does mention the urge to you, suggest having her hold an ice cube until it's completely melted. While she does that, either have her come to you to help her find something distracting (comedy movie, good music, funny story, whatever) so that when she's done with the ice cube the urge is more likely to have passed. The ice cube will "burn" but it's better she does that than cut herself.