⚠️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, 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.

Update???:

So, she wore short sleeves today and the marks were GONE. There's no way they healed so fast, right? Unless maybe she didn't actually break skin??

I feel like I made it all up now and I might be spiraling. Any proof of her self-harming has completely DISAPPEARED and she won't open up to me about how she's really feeling.

She just keeps making jokes in passing about 'crying myself to sleep', 'wanting death', and 'being emotionally scarred forever'. I genuinely can't tell when she's making jokes and being dead serious. My mom is acting like nothing's wrong while I'm in my room spiraling over nothing.

I don't know how to talk someone about this because as I said, there's NO PROOF ANYMORE! I just want to let go of it, but something is telling me to talk to my mom. How do I say I'm worried when I can't even show what I'm worried about?