Always be there for her. Does she cut herself simply because of her mom's condition? If so, ask her why she is risking time away from her mother like this. Treat her with love and hope for the best. Talk to a trusted adult if neccesary- you won't be snitching, you'll be helping her.

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.

Tell her that her mom wouldn't want her to hurt herself like this.

Your best friend is scuicidal? Join the club.

Well Im going to say dont ever assume people wont do something, because I have just lost a lad friend who jumped in front of a oncoming train as he was severely depressed. Unlike your friend he never told anybody about his upset and feelings so I would reccomend a counsellor or such like, as I think something so serious should be left in the hands of the professionals.

i agree with magic&madness! you should tell her mom! shes really weak, and she probably feels like once she looses her mom, then she wont have anything, but her mom will always be there for her. so you should tell her mom. she needs to hear it from her. as well as you