Sister is suicidal!! HELP!!!

My sister is super suicidal right now. She is listening to songs about it and everything! She even told me she has had plans. My mom most likely will refuse to get her help and I know my sister will fight it. She is 17. Is there any legal way I can get her help. I would have to fight my mom with it and fight my sister to get help. I've been where she is. I know how she feels. I've almost done it myself! I really need help!! PLEASE!!!