Whilst most teenagers go through a difficult stage, which can actually often include thoughts of, or even half-hearted attempts at suicide, this sounds a lot more like a psychological issue that needs to be dealt with as soon as possible Dilmah, not only for her sake, but for yours also. Dare I say it, but it is selfish of her to hurt you like this, because it is not only her own life she is damaging, it is yours as well, especially when it is in the middle of your exams. It sounds like you have devoted a lot of time and effort on her behalf, and it is not unfair to ask for a little effort on her behalf in return.
Personally, I think this goes beyond a 'cry for help', especially if it has been attempted before, is there any way to talk to the hospital to ask if there are any other options open to you for helping her other than simply repairing the damage afterwards?
Either way, I am glad that things have, kind of, turned out ok, and hope your sister is feeling better soon.