Why does my mom treat my dad and I so poorly, but treats my sister like an angel?

My Mom has always treated me as if I was the unwanted child, and lately it's become the same way towards my father. She has repeatedly told me to kill myself and told me and my Dad we are lazy and worthless, but will takes time out of he day just to play and compliment my young sister. My Dad has done everything possible to make her happy, yet she treats him like he's nothing to her. She constantly puts us down and tells us she'd be better off without either of us. I don't think I've seen her fold clothes, do laundry, wash dishes or clean the house since I was a toddler. Her mess is left for me and my Dad to pick up. I've heard her argue countless amount of times with my Dad, but she's the only one screaming. She tells my Dad that she wishes she'd never stepped foot near him, that marrying him was a mistake. He refuses to yell back or insult her even when he comes out of the room nearly in tears. Sometimes I want to confront my Dad, tell him that this isn't a healthy relationship and he needs to leave her, but I know that he'll stick up for her. He's too nice to be with this rude lady who I don't even call a mother anymore because she claims to not want me. On a few encounters she has hit and kicked my Dad. I'm so lost of what to do and i'm not of legal age to move out or do anything beneficial to majorly help. I want so badly for my dad to have someone who he deserves and I can't stand him being accused and insulted for things he's never done. Around other people my mother is like any other person. Nice, likable, easy to get along with, but when only me and my father are around the tables turn, we are shouted and screamed at. I honestly can't tell you how many times I've asked friends to come over or come along on a car ride just to not hear my mother shout at us for at least a half an hour. Morning, Noon and Night there's yelling. I don't have a clue what to do anymore, I feel like she's like this because of me. I never remember her being like this when I was younger. Please help, I want something to change for the better. I'm worried for my Dad.