It's not an easy situation to be in....I apologise if I sounded a bit harsh before, I didn't mean it that way. I really hope your sister gets better and that you keep well.
Remember that the only thing that can always be offered is love..And it is a big thing! It sounds silly but it is so true, especially when all words of advice fail, because of the state someone's in...So never think that you're not doing enough for your sister...