crazy big sister?

So I babysit for a 4 year old girl named Makayla, and her 7 month sister Riley. Makayla hates Riley. Anytime they are near eachother you have to watch them closely. Makayla is currently on time out for holding a blanket over her baby sisters face because she was crying. Anytime Riley cries, Makayla will hit her and say she's a baby. Makayla has hit me, kicked me, pushed me because I made pizza rolls instead of pizza. Do any of you guys have any ideas? I wasn't sure where to post this. But I'm super frustrated and this little girl is basically the devil.