My little sister is openly harming herself

Im 20f. My 15yr old sister has clear self harm cuts on her arms and thighs. She doesn't try and cover them up at all. I haven't said anything about them because I genuinely don't know what to say. I wonder if she wants someone to ask about it, but at the same time, she's an extremely moody and combative teen. You have to walk on eggshells around her because everything makes her blow up. I was very similar as a teen but I quickly grew out of it after highschool.

I have no idea how I would begin to approach her about this without making it way worse. It makes me sick to imagine that she's in so much distress she's doing this to herself. I don't want her to be in pain. This is also causing me crazy amounts stress because I'm already constantly busy with work and school. I wish my dad would be more emotionally available to help her but he works most of the day because we live paycheck to paycheck.

Does anyone have any advice about how I should talk to her?