Why does my mom blame every single family problem we have on me?

I am a 15 year old girl. I have depression just so you;d know and my mom is like 40 something and she blames every one of our family problems on me. Like my dad is terminally ill and could die any time now and she blames his illness on me and that I am putting more stress on him and going to kill him. Also that she is going to tell everyone at his funeral that I killed him by adding stress which I don’t she does. I have lots of stress in my life right now school, my dad might die, my annoying 8 year old sister and I know everyone thinks there little sisters are annoying but mine gets everything she wants, when she gets mad and hits on me or talk reallly bad to my mom, she never gets in trouble my mom just yells you know she wouldnt be so bad if it weren’t for you. She never punishes her she punishes me for what my sister does wrong. She makes me feel worthless and she cant go one day without yelling at me or she’ll go crazy which id dumb because even if I am as god as ever she has to yell at me for something just to get to me. She calls me the worst names ever and gets in physical fights with me.I can’t stand her anymore. When I try to get away from her and tell her I don;t want to fight and go to my room or somthing she just follows me and just keeps aging it on. When I was told to tell her that I don;t want to fight and go to my room she does this so how do I get away from her when she does that. PLEASE HELP ME I DON’T KNOW WHAT TO DO.