Teenage sister is going off the deep end. How do I help?

I am really worried about my 16 year old sister. I always noticed that she’s been very deeply emotional - very sensitive, angry a lot. We also live in a toxic household but CPS has never helped us so we all just struggle through. I just got out a few months ago because I had a baby. Well recently she told me she started cutting herself, which I (for some reason) automatically envisioned a razor and light scars. But my mom tells me she took a huge kitchen knife and started hacking at her arms. Has at least 50 huge gashes. She said she did it because she argued with my dad (he’s an alcoholic and toxic af). Not to make it about me at all, but my heart is so broken for her.

On top of it, some things she says don’t really make sense. She tells me that my mom doesnt care and wouldn’t talk to her - she’s told me this multiple times. Said she even made fun of her. I thought it sounded VERY unlike my mom but I had no reason to disbelieve her. But I asked my mom and my mom says that she has tried to talk to her many times and my sister shuts her down. That version is more believable to me because my sister does not like talking about emotions. She had a virtual therapist but on her first session said that she didn’t want to have to tell her her feelings and problems so she just randomly left and refused any further correspondence.

Last night she said she wants to talk to me because she’s been feeling really lonely. Since she doesn’t open up, I’m scared that I will slip and say something she doesn’t like and she’ll go quiet. What do I do?? How do I respond if she mentions my mom?? What can I say??