All that you can do is what you are doing, recommending that she get help and being there for her. So you are doing the best that you can do for her. Try to explain to her, though the other siblings do need help, that this is a situation where her seeking counseling would help her deal with them. That might work. I don't know if she will listen or not. But you have done all that you can do for her, and now it is up to her to take the next step.
I wish you luck,