I have fallen out with my mum and sister?

I didn't want to fall out with them but it just happened.

It happened because my mum had given my sister a sky cable box with amazing tv channels and she would not allow me to have one. The reason I got angry wasn't because of the tv thing but because she gives one daughter one thing and not the other.. Allowing my sister to smirk at me and rub it in my face. She told me I get everything but I never ask her for money as I make some myself from selling things. What really annoyed me is that my sister hits my mum, shouts in her face, throws things in her face, calls her a 'bittttch' and whenever my mum goes out my sister will scream and scream. I have to babysit my sister, I let my mum borrow money from me, I let her borrow my clothes, whenever it's my mums birthday I am always the one to plan nice gifts and I also sometimes bring her suprise gifts for no reason, I let her friends borrow things from me, and whenever anybody says anything bad about my mum I always stick up for my mum. She doesn't appreciate this at all and somehow sees me as the bad guy. I don't feel welcome in this home anymore. Today I pushed my sister out of my room as I had told her to get out like five times, she told my mum I was shouting at the baby that was in my room and I wasn't I was shouting at my sister to get out, so I pushed her out of the room and she comes up to me and attacks me grabs my ear and scratches my face and continues to pull my hair.. I got slapped across the head five times from my mum. I don't find this fair and I've had to ignore them because I am getting tired of this. My mum told me to just leave, that made my heart drop a little bit because it makes me feel even more unwanted. I always have to sit in my bedroom alone! I am 17, my sister is 13 and she basically rules my mum. I don't know what to do as I love my mum but I can't have this happen all of the time. What should i do? I've written her a letter but I doubt she will understand as she never does.