OK. First of all I want to say that you are a very good person and I wish more people were like you today-caring. You rock.
On topic, I think this is gonna be long so I apologize to everyone in advance.
I've been in a similar nasty situation like your sister when I was teenage (which I won't describe), but it's more than enough to understand her cause I acted the complete same way, without the writing of course since I am no writer. Yes, she is depressed. I'll try to explain something: as a girl, you don't necessarily need to get raped to feel raped. Why? Sole the fact that your body is "abused", (someone starts touching you without your permission, and you have to repeat times and times to stop, and he stops, but then it happens again, and again) makes you feel like your free will is taken away. On top of that, as a girl, you get to realize on the painful way the most painful lesson in life: that at no matter what age, men will (almost) always be more powerful (physically) than you. Trust me, I know what I am saying, I was very young too. The feeling of not having control over the moment when someone decides to touch you and you just don't want to be touched! And it doesn't matter if they were "in love" or something, abuse is abuse, when you don't feel like it - you just don't, and when someone forces himself into your body, you scream on the inside. Helpless. That's the feeling a girl gets in time when is faced with that kind of experience. Plus she was way to young to handle touching like someone trying to put hand under her shirt. Woman at every age can find that extremely disturbing if she is not prepared and willing to such an act! It's a direct insult on ones intimacy, it leaves scars more deep than one can understand. And of course she can't be hugged now, even from your parents. The wound is too deep, and even when hug is coming from a loving parent, traumatized person feels abused again: someone is reaching for her again without warning, she couldn't see it coming, and someone is over her body again without permission. And she feels enraged. I use to act the same way, completely identical, and that is cause once you are hurt you see threat in everything. It takes time to recover from that, and therapist can do things for her, but she needs to come at peace with some things for herself. Be there as a support, if you think that you can talk to her without blaming her for the behavior you'll do her good. But stay away physically, you don't need to hug her to show her that you care, cause hugging for her now has a bad meaning, and trust me it doesn't mean that you care. And maybe for a lot of time to come she won't be ok with hugging, and that is fine! She'll do it when she is ready, and no one, NO ONE should tell her and make her do otherwise! She'll get better when as she grows up: time will do it's part, caring people their part, and hopefully a boy(friend) if she allow herself the luxury of falling in love, that will treat her with care and love.
And the thing with withdrawing... normal. Being introvert after that is also normal part of it (as is being enraged extrovert). Good thing is she writes, and is channeling her emotions through there. Maybe she leaves you hints deliberately with the open writings, maybe not. Keep in mind that she is very young and that she even doesn't really understand herself, and this emotional burden is way too heavy for her to carry at the age of 14.
I was "emo" too back in those days, and then I started to listen to angry metal bands cause the anger consumed me. And when I was tired of rage, I listened to some pathetic tunes and cried for days, and than again the anger and so on and so on.
You need not to blame anyone. Yeah, the guy did it. Maybe someone should explain what damage he caused to her, I don't think he is aware what he did cause it was not in his intentions to hurt her. So he needs to learn the lesson from this too.
Sorry for the long reply, it just hurts me a lot to see things like this.
Good luck to you, and to her, I wish her well recovery.