Why do i me and my parents always argue when i'm home from university?

So basically I'm in my second year at uni and have moved away from home. I love it as I have a lot more independence and can do my own thing. However, when I come home in the holidays I struggle as there's not a lot to do other than work at my part time job and I live quiet far away from all my friends. My parents and I are fine when I am in uni and I talk to them on a weekly basis but when I come home everything is so negative. I have a 16 and 5 year old sister and all they do is argue and my parents just get so wound up about it and everyone is constantly in a bad mood. They complain that I'm always so quiet and never smile but to be honest I hate coming back to this sort of environment. I treat my 5 year old sister like my own child and I understand she's excited to see me when I'm home but I can't do anything with friends in my house when she's here and she's constantly on top of me when I'm tired after a day at work. I always babysit when I need to and help with her during the ween but My parents constantly say how ungrateful I am about it and that I never do anything but I'm always looking after her and I feel like they need to understand I'm only 20 and don't want to be looking after a child. Furthermore, due to me living so far away my friends constantly have to drive me everywhere and I struggle getting to work as the transport here is so bad. I passed my test 2 years ago and my parents refuse to cooperate on me being able to drive a car as they say it's not worth me just driving for the fun of it ect. I had an argument about it this morning and they just went crazy at me saying that I'm so ungrateful and I need to stop feeling sorry for myself all the time. I said that I have to work until midnight tomorrow and how will I get home and they told me to just get a taxi which would be so expensive. They just don't understand the guilt and stress I constantly have to go through trying to get lifts all the time and it's not fair on other people. I never ask for lifts as they just moan and say "am I a taxi". I'm constantly crying when I'm home as I just don't feel like I can do anything right when my 16 year old sister gets away with everything and can just stay in bed all day. It's sad because I hate being home as all my independence is taken away and everyone is just so negative and I feel like they just turn it on me. Plus I said I'd pay for insurance but they said they'd rather keep their Range Rover as they buy cars for themselves not for us when I asked why cant we just get another smaller car but apparently they can't afford it? I'm just so fed up as I feel nothing I do is right.