Hi everybody!

So I live with my mom, my sister and my sisters 2 kids. One kid is 2 and the other is 4. She does not listen to anyone! Not her mom, or my mom, or myself. My sister recently got a job and asked me to watch them. My 4 year old niece does not listen to anything no matter is you yell, are calm, try to show her the right way, nothing. I have worked at a preschool as well as a school for kids who have autism, which my niece has, and I have never had this hard of a time. She laughs when you yell at her and even laughs when my sister has spanked her. I have tried everything and every tactic I know. She hurts her brother and does not listen to one thing you say and continues doing whatever you asked her not to. I am so lost. Please anyone with advice please help!