Opinion on family issues?

This seems like a strange question, nothing is really black or white, but this always bothers me so much.
I am defensive of my mom, but I feel torn because I feel more like taking my sisters side for this. I am pretty stressed.

My mom and sister fight a ton, today I was woken up to it. Sister asking my mom for the $70 she borrowed back, but my mom, though she said she would, wont sell some of her stuff to get it to her.

My sister owes her money for the phone plan as well, which I think she should pay, but i dont know how I feel about my mom just withholding my sisters $70 because she owes her money from elsewhere when that wasnt agreed to beforehand.

Anyways, my sister is 18, doesnt do anything around the house, is quite dirty, but I figured that my mom should either give her like 2 weeks notice and either see change or have her leave, or cut off her phone. Quick, simple, she will be pissed but my mom would assert her control.
Instead they always scream at each other, and something about it rubs me the wrong way a ton. They call each other 'retarded' (a word i cant stand the improper use of), '****', 'psychopath', '*****', 'dirty whore', 'garbage', 'useless'. My mom using most of those words, my sister just kinds yelling ***** at her and stuff.

But I don't know how I am supposed to feel/ what I am supposed to do. I am only living at home temporarily, but i feel i need to do something? Is this normal?