I hope your sister is already getting some help, but if not, I offer this advice. My sister also had some severe depression issues in high school that my mom, like yours, didn't feel were a big deal or anything to worry about. My sister ended up not getting any professional help (we all were sort of working through personal issues so we were all pretty tuned into just ourselves and our friends rather than family) and is actually o.k. now, although she would have been much happier with some help at the time.
Anyhow, my point is that my sister, who is now in college, asked my mom if she realized just how miserable my sister was at the time and all the horrible things she was experiencing. It turns out that our mother did not realize the severity. Telling your mother about the cutting might help her get the point and even your sister showing her the scars might scare your mom enough to take some action. Perhaps your mother is experiencing similar feelings and feels getting help for your sister would force her to deal with them.
I understand also about the school guidance counselors not being of much help. My best friend was verbally and sexually harassed for most of high school only to have counselors tell her "boys will be boys" and "boys will tease you and the like if they have a crush on you."
However, there are many counselors who do care and could really help. Maybe you or your sister could talk to a different one or even a teacher or other adult you trust? Perhaps if they talked to your mom she'd take action. She might act defensively at first but the point might get through to her.
In any case, you are good to want to help your sister and it is excellent that she is trusts you enough to talk about this. I'm sure knowing she has a caring (older?) sister helps her a lot.
"I'm a cunning linguist" ~Princess Superstar