I am tired of babysitting my younger sister.?

I am 18 and my sister is 9. I have babysat her for the last four summer breaks every single day from the second i wake up to about 5 p.m. I even have to watch her anytime my parents need to run errands. I also have a job and work the evening shift 3 to 5 days a weeks. So i hardly have time for anything. This summer I have had it, but I don't know how to tell them. My sister doesn't listen to me at all and just sits inside all day watching tv and playing computer games. I have a girlfriend that I really like and friends I would love to spend time with. I have attempted to talk to my parents and even offered solutions such as summer camps and friends that babysit as a job but they always say no. My family is a member of the YMCA but my parents will not let her do their camps either. I know my parents have the money so that isn't the issue. Anytime I mention that I don't want to do this to my parents they get really upset,ignore it completely, or act like I do nothing to help the family and guilt me into doing it longer, they also have a tendency to blame my girlfriend for everything that I have done or asked for that they don't agree with. They won't even attempt to get to know her and she is to the point where she has quit trying to get to know them. Her parents have invited my parents over to their house a bunch of times but my parents always make excuses and don't show up. This is my summer before college and so far it has been horrible. I want to take control of my life and have some say so in the things my parents want me to do. I would love to be able to go out during the day and enjoy myself with my friends and girlfriend like a normal 18 year old and not be tied down every second of everyday. Can anyone help me?