You are a good sister to care about your sister and want to help her. It sounds like you both have grown up in a difficult environment, and since she is still living with your mom, it is still ongoing for her. You didn't say what it is about your mom that makes living with her hard, but even "little" things like invalidation and being controlling have a big impact.

I am glad that both of you are getting some professional help, but it sounds like you are even having something of a struggle there with getting the kind of understanding and support that you need. Sometimes you do have to do some shopping around to find the right therapist for you. One thing that you could do is talk to your sister about her therapy. Ask her if her therapist knows about the cutting, and other things that are important that she might not be sharing, and encourage her to talk to the therapist about those things. It is scary, but we just don't get anywhere with therapy when we are keeping secrets. You might even ask if it would be okay for you to go with your sister to one of her therapy sessions. Your support might make it easier for her to open up about things she has not been able to yet.

Even if self-injury is a way to get attention, there is some reason why that attention is so desperately needed. Sometimes I think cutting can be a way of communicating, "hey, I need some help and I don't have a better way to ask or to show anyone how bad I am feeling!" Smiling and giggling while showing you or telling you about it don't make it less serious - that is most likely a sign of nervousness. The good news is, even if she has been thinking about suicide, she has stopped short of that and scratched or cut herself instead. She has not given up, but has found a way to deal with her desperation instead, even if it is not a healthy way. Sometimes SI is an alternative to suicide. There are so many things that it can mean, and every person is different, but whatever it is that is compelling your sister to hurt herself, this is something serious and too big for her to handle by herself, and she does need support. You do not need to take on her burden yourself, but she needs to be taken seriously and making sure that she is getting the help she needs is the best thing that you can do.

<font color=orange>"If a light beckons to you, follow it. If it leads you into the quagmire, you'll probably find your way out of it again; but if you don't follow it, you'll be plagued for the rest of your life by the thought that perhaps it was your star." Friedrich Hebbet</font color=orange>