Don't let your sister ruin her body with cutting. I've been trough this and now I regret it.

OP, Honestly, I'm not sure how to feel about this FML. I want to spout on about how you did the right thing by telling your mother, but I also want to look from everyone's perspective, including your sister's. You may have found out something important to know, but how would you have found out unless you were snooping? And your sister would feel as if her privacy was violated if you told her mother. You should have talked to her about it before telling your mother. It would have been embarrassing for her, and telling from your mother's bitchy reaction, she probably would have gotten pissed if she believed you. Pissed at your sister's wanting to express herself.

Get her some help and be supportive. She only writes it down because she has no one she can trust with that stuff, be there for her instead.

Tell someone else who will believe you? Other than that, support your sister.

This needs a follow up

I know this is late, but this definitely needs a follow-up. Also, if anything like that happens again, I am really sorry, but it's kind of a shitty thing for your mom to do. If anyone has even a shred of evidence that someone is thinking of or actually harming themselves, get that news to parents, guardians, spouses, or whomever they can. It's a pretty ******* serious issue.

go and get a professional then. This isn't something you should ignore...

your mother is an idiot.