I've been seeing semi-deep cuts (not bleeding or scabbing, just not regular scratches) on my little sister's arms and legs over the past few weeks. I may very well be overreacting, but I'm concerned. Any advice on how to approach this?
If asking questions would help you understand the situation better, please do (of course there are some I can't answer).

I realise it sounds odd but to be honest with ypu I wouldn't. If you're really concerned to the point where you need to do something, tell her she's seemes sad lately (in private!!!) and see if she tells you anything. If she does then you can help her. If not (especially if she says anything to the tune of "no I'm just tired") just tell her that if she needs anything just ask. It doesn't sound helpful but as someone who suffers from depression, I can tell you that knowing someone is there for you unconditionally is one of the most helpful things.

What confuses me is that she doesn't seem sad or tired at all. She acts just like she did before I saw the cuts (energetic, social, laughing, joking...). Also, she is being quite conspicuous about them and I know that she could very easily make them less noticeable.

she probably wants someone to figure whats going on with her then, she doesnt wanna go about mentioning it first. >>2 might be some good advice to start with.

Well I found out today that she has been trying to be "emo" (quotation marks due to my not fully understanding what she means) and has been scratching herself with a pencil because she thinks it's cool. So, new question: any suggestions for how to get her to stop? I know I'm probably being overly protective, but asking her does nothing action-wise and graphite isn't great to have in your skin.

Ok here's the thing...
First off you're not being over protective, so you can scratch that worry off of the list. Secondly, since that is the case cleary she is being misguided and misunderstanding the whole "emi scene". Thirdly, emo is a slang term used to identify and vilify those that cut themselves due to emotional/ psychological trauma from the past or present. However, there are some, as you sister seems to be doing, do it for attention. I recommend that you bring your parents into this, if haven't already that is.

My mom has noticed, but hasn't gotten any further than I have. Telling my dad might scare her out of it, so I'm not going to do that.

Maybe then just you and your mom discuss it between yourselves? I don't know how she got the idea that it's cool because it's definitely not. It could be the case something has happened or attention but either way it needs to be addressed. Maybe a school counselor but that might scare her off too idk.

Emo's do not cut themselves, that stereotype pisses me off. I'm not emo, but I've researched about it. They are a group of people who listen to a genre of music that is Emotional/Emo. They also dress in the style of that genre which is dark colors for some of the different styles. But they are not a group of depressed and sad people who cut themselves to relieve pain or get attention.

Perhaps you could introduce her to the story's of why others used to cut themselves? If you could give her a sense of the gravity of the situation, of how purely wrong and frightful cutting really is, maybe she could back off.