How do I show support in an already strained relationship?

[ealaurel]

My sister and I have never shared a typical loving relationship, no blame to either party. We are 18 months apart, which at our ages (17 - myself, and 18 - my sister) already creates a lot of strain, being adolescents and in high school and all. As of this past year, however, we have been taking baby steps towards a better relationship. She has let me see further into her personal life, and has begun sharing more specific details about her interactions with people - how I know she is starting to trust me. I, on the other hand, have grown to care for her more than I could possibly imagine, leading me to find sites such as her twitter account and tumblr in order to see how life is really going for her. ** I realize that this could appear to be a violation of trust and privacy, but it is important, as it has lead me to joining this forum and seeking help **
What I find on these sites are particularly upsetting to me. I have known that she has never been completely content with her life; she has always struggled to keep constant friends in her life and overall to socialize, she doesn't try in school enough to support her high intelligence on a college transcript, and my parents constantly on her about it.
I had no idea that it was so extreme. I have fully realized and accepted the fact, though, that she is depressed. And I just want to help. I was brought to tears as I read some of the messages she has. I had never known I cared so much, and realize that I truly love her more than I could ever even fathom. To see that not infrequently she wishes herself dead, that she feels alone, just wishes that she had a friend to care for her - it breaks my heart.

I want her to know that no matter what I will always be here for her. I love her, I care, and if she were to ever be harmed or worse, I don't know what I would do. I want her to know that I am always here to support her. I care. I don't know how to tell her. My sister and I have never exchanged the words "I love you" with each other. We have sincerely shared one hug. I am scared to speak with her, as I don't wish to anger her or offend her in any way. I really am at a loss at how to approach the situation, and any offerings would be appreciated to no end - when to discuss, what to say... I am afraid to wait too long because after this "epiphany" I fear what could happen at any moment. Please, please, please help me.

***For a brief history of my sister's background*** my sister has never been officially diagnosed as depressed, though It does not take a genius to recognize. My parents were made aware of a problem she experienced in middle school, where the school councilor contacted them about his concern over her behavior. While I am not sure of all of this, as I was deliberately kept in the dark, I believe she resorted to self harm during that period of time. It was not extreme, but self harm is self harm. She has done nothing I am aware of since. My parents, though, do not help the situation at all. Our relationship with them is quite similar to that we have with each other. They pressure her about school (thankfully it's summer now) and do not seem aware or in acceptance of the full extent of her situation.
.I am quite sorry if anything in this post has crunched toes or hurt anyone in any way. As I briefly mentioned, I just made this account and I am not fully aware of what to be careful of and what to avoid yet (despite reading the guidelines), however I try to be as empathetic as possible.

[fallen]

the fact that you love your sister is a beautiful thing, and that you are trying to find ways to help her is good also.
all you can do is be supportive of her in general and around your parents so they don't put to much pressure on her, and try and hint in a subtle way in time to maybe talk to someone a professional about her feelings.
there are other people who may be able to give suggestions, or try the chat site where people talk about all kinds of things but they seem to be your age group and you just have to jump in and ask for help on your sisters behalf.
take care