Please help! Depressed little sister?

Okay so my little sister used to self harm...this started happening from the time she was about 11 until she turn like 13. I eventually got her o stop all on my own just by talking to her and letting her know that I love and care about her. As time went on I noticed that she still wasn't very happy and though she was no longer hurting herself, she was still very much hurting inside. I did some research and found that she might have bipolar disorder or perhaps some Other form of depression. I'm no doctor and certainly don't have he degree and education required to diagnose her, but I do know hat my Little sister is not the way she used o be. Anyways, she talks to me and we have a great relationship having been only a year apart in age, but the probably is that she talks only to me. She won't let me ell our mom or anyone the way she is feeling and she doesn't want to do it herself because she feels embarred. I can' help but notice that it's only getting worse with time. Lately she's hardly been talking at all to anyone about anything. It's heartbreaking to see her this way and I just need so advice on what I should do. I don't know if I should tell someone or what. BTW I'm a 15 year old girl and my little sisters 14 Please help! And thanks in advance, I know it was a lot to read