I hate going to my dad's house and no one is trying to change the custody agreement. What can I do?

My dad is very irresponsible, and when I go to his house, I have to play the role of parent for my 11 year old sister. I am the one who usually has to do the cooking, and I am have to oversee that she does her homework, etc. If I did not step up to the plate and do these things, they simply would not get done. My dad has repeatedly been drinking and driving with my sister and me in the car/ motorboat, and I do not at all feel comfortable with this. My dad and I do not have a good relationship, if any, and I am extremely tired of being forced to go to his house. I am afraid that if I don't, there could be legal consequences for my mom, but she has told me that right now she can't do anything to change the custody agreement. I am really stuck between the legal and emotional aspects of this situation and I have no idea what do. All I know is that I face great anxiety when forced to go to my dad's, and that I really can't continue on with this. What should I do?