What should i do about my nephew and my sister?

im 17 and my sister deedee is 33 and she loves to party. every weekend she calls me and tell me to watch her kids(my niece and my nephew). i never want to do it but i always do it because i always need her for everything but now she always lie and say she has to do overtime at work at work and i need to watch them because she needs a babysitter. i know she is lying but i do it anyway. my nepher christopher is 10 and my niece chase is 2 and me and christopher argues all the time because he acts so stupid and dumb and takes me for a joke. i always tell him to do something and he never does it and acts so stupid laughing taking me for a joke. so i get frustrated and i punch him up a bit and fuss and fight with him. my niece chase is ok but she curses a lot and she is only two. im tired of watching them because my nephew is a evil little son of a gun. my mom tells me to stop watching them because thats my sisters responsibility but i do it anyway. what should i do???