Understandably this girl seemed to have enjoyed coming to this site, and concern is naturally going to be the case. But in my honest opinion I think you should rather BE with your family instead of hanging around online. If you need to understand what is going on with your sister you should focus on her REAL life, friends, school peers etc. That is the only way you may get closer to what the issues are. This place, the net, is generally like Hollywood... great fun, escapism, fake for the sake that it's a plastic thing with a screen and one can genuinely just indulge in losing oneself in all the information, conversations and lack of responsibility. But this is NOT the place to get to know your sister better. Sure there is the information on cutting, read that, but I'd say speak with the hospital's psychologist, whom I am sure will be speaking with her and your parents without a doubt.
Just to clarify something, she was at your house, left the comp and DF open and then went home and slit her wrists? Or cut extensively as it were....