Why is my mom like this?

okay so my younger sister is 11 and she has adhd, gets in alot of trouble at school, and is very violent with everybody. My mom treats her like royalty. She gets emails home every single week about my sisters horrible behavior and how she disrespects teachers and is rude to kids, shes even on a special system her grades are basically all F's too, my mom always gives these excuses saying the teachers pick on my little sister and that shes actually super smart. My sister at home violently and verbally abuses me and im 15, a girl too. My mom always says i make fun of her so much it ruins her and thats why shes bad at school, because ive tormented her for so long, ive done nothing. THEN my mom tells me ( i am also the oldest two younger sisters) that i'm an ungreatful whore and a misreable person, i have an excuse i was raped two years ago by some kid and yes i have been very depressed too, she doesn't know i was raped and although she still saw i was cutting myself she almost made fun of it saying things like "well at-least i dont cut myself" it hurts so bad because she has no idea. She treats me with no respect and acts like i do everything so wrong and my youngest sister is amazing, she rewarded her with an ipad and a limousine to the airport when she got an A on a TEST. it's ridiculous, she's spoiled her, and i am not like that i barely ask for new clothes. My middle sister agrees with me too, i am just wondering why is she like this?