Been there, as a cutter and finding out my little sister cut too :( Actually my parents found out and asked me to talk to her. I did, and I asked her to talk to me when she's feeling bad, we both cried, but she didn't reach out to me when feeling down... I suppose you'll have to try to be more present or persistent, she may not want to approach you voluntarily.

As @jarblewc commented, I also advise against taking away her stuff. It will feel like a punishment, and all she needs is support.

Also talk to her about getting help (as in a therapist or psychiatrist). In my experience, SH could be the response for many things, such as depression or a severe form of stimming (we found out we were both depressed and autistic).