First of all, she's pretty (^_^). This is an unfortunate situation
. I think her sister is blaming her because that is how she copes (so perhaps she could use help too). Honestly there isn't much you can say, you haven't gone through it before and even then you know all you can do listen, and comfort them. My best friends mom died of cancer in front him, what do you say to that? Make sure you're always available to talk, even if you're doing something else that's important to you (sounds like you are already doing this). The cutting is something she's going to have to make up her mind on, just like if she blames herself, etc. Supporting her, telling her nice things about herself and all that good stuff should help. This is just my personal experience so... you gotta read em before you do anything.