What the hell can I do [Narcissistic mom]

I have a narcissistic mom with 0 empathy so she doesn't care about our feelings and will never understand us and only thinks about her own feelings and hijacks every argument into her own hurt feelings no matter how crazy my sister gets whenever we argue. There are times she's held a knife and tried to hurt herself when arguing and I scratched my neck and skin when I started crying because she pissed me off and all she did was looked at me with no emotion.

Recently I'm trying to pick up the behavior of not arguing back with her or giving her the cold shoulder because I was told that's what you should do to narcissists who provoke you. But... my eldest sister has a big sense of jutice so she keeps feeding gasoline to the fire and it just turns into an endless loop of fighting and my mother trying to make a "point" with her feelings.

I know eldest daughters have a baggage I won't ever carry or feel, but being in the middle of both of them is so draining... I have to go through my sister's episodes and try to help her not hurt herself and keep her on her feet and hold myself back from my mom's harsh words which she deems as harmless. I'm just getting tired of this loop but she just keeps feeding into it. I told her many times to stop.. but everytime she says she needs to let it out or else she'll explode and at this point or worse she fights me back as if this is her fight... sometimes I can't even jump in because it feels like I don't have the right to as my mom doesn't see me as important because she relies on my sister too much and I don't suffer enough like my sister.. like maybe I have to easier than everyone else.

Now I'm thinking should I just leave them be? It feels like I'm not any important to them or make any significance in the family and she just includes me in their argument so I won't feel left out, but what if she secretly believes she's the only one suffering?