<human>: We were talking in her room when I saw the scratch marks. I was really shocked and asked her abruptly what the scratch marks were and where they came from. She got defensive and responded with "nobody, and it wasn't me, just don't worry about it. I will tell you when the time comes." When I asked her if it was by an animal, she said "no, it wasn't our dog, just nobody, and it wasn't me, just nobody."

I'm really worried and freaking out at the moment, and I'm thinking of the worst possible scenarios as she is a freshman in high school and it is highly possible that she could be hanging with bad influences among other things. For the past couple of months, she's been reblogging 'depressing, grungy, gothic and edgy' fashion looks, quotes and photography on her Tumblr blog.  Although this interest of hers is not reflected too much in her behavior, I feel like this might have something to do with the scratch marks.. She also does know what 'cutting' is because she's seen it on weheartit and tumblr. 
I don't think it has much of anything to do with an abusive friend or boyfriend, as she doesn't have a boyfriend and she always comes right home after school. 
We do have a small lap dog but those marks do not look like our dog's scratches and they are much too spread apart and in different directions to be that of our dog. She also says it's not from the dog, as I mentioned above.

Right now she's trying to make me feel better by trying to joke around and bring up inside jokes that we often share, but she won't talk to me about the marks. I just can't imagine what kind of events had to take place for her to have those marks on her arm.

Reddit, I'm scared.. what should I do? I love my sister and I can't stand to see her hurt.. I wish she would open up to me, though I understand my confrontation reaction wasn't helpful. Does anybody have any thoughts on what she might be going through in high school? Has anybody experienced this with their younger siblings?

Can you condense the text into three sentences or less?
<bot>: My sister has mysterious semi-deep scratch marks about 1.5 inches at length. I'm shocked, and she refuses to tell me where they came from.