How do I tell my mom my sister is self-harming???

⚠️warning: self harm/suicide⚠️

So, over the weekend I noticed my sister had cuts on her inner arms. She had her sleeves rolled up bc she was cleaning, so it was pretty visible. I don't think she realized I saw, but I did.

It looked like she scratched herself really hard, and like she did it a lot. She's been wearing sweaters/hoodies a lot, so it makes me think it's been going on for a while?

I know I shouldn't, but I feel really guilty for not knowing sooner. But now that I'm aware of it, I don't know what to do?? I'm 14 and she's 12, so I don't have a lot of power here. And our mom isn't the biggest... believer (?) in mental health, so I don't know how to approach her to talk about it.

And I can't go to her dad bc he isn't in the picture, so I feel like I'm in a lose-lose-lose situation. The internet says to talk to her about it and support her anyway I can, but she shuts down or deflects the conversation whenever I even mention mental health.

What do I do in this situation? Should I go to my sister directly, try and convince mom this is serious??? I don't know where to go from here.