Brothers treat me like a slave! Help!?

My big brothers are 14,19,21,29 and they mind me when my stepmother and father are at work. Im the only girl and the youngest. My brothers always play silly jokes on me with their girlfriends or friends. They make me clean up everything. Their bedrooms,(and they each have their own so that's 4 bedrooms) clean their clothes, and basely act like a slave. My stepmother should be doing all this but her mother just ped away so I don't want to ask her. My Father works in a different country. I told my stepmother what they are doing and she told them to stop and they do stop when she is there but when she is at work they start again. What will I do?