Annoyed by sisters daughter

Okay so my husband and I live with my parents for a bit while we look for a place out of state and my father watches my niece who is 6 almost everyday and I have a 3 year old daughter myself. Ever since we moved in and we are around my niece everyday i seriously cannot stand being around her and I know it probably sounds bad bc she is 6 but she is mean to my daughter and then my daughter copies how she acts or what she does and I hate it. My niece talks back, screams at people, is intentionally mean or rude, throws serious fits when being told what to do, doesn’t listen, ect. I can ask her to please not do something and she will completely disregard what I said and just ignore me or even cry and throw a fit. She will talk so meanly to my daughter and will throw a fit if my daughter has something she wants, “accidentally hurt her” and then scream “I didn’t hurt her” when my daughter comes to me crying which makes me think she is doing it intentionally. She will tell her mom (my sister) or my father that I’m mean to her if all I’m doing is telling her not to do something or telling her do something like pick up her toys after she’s done playing. She is never really made to listen by anyone and doesn’t ever get disciplined other than being threatened with not doing something she wants to go do or to go to bed but they never actually do it. Anyways I’m annoyed af bc my daughter is learning from her and I hate it. My daughter loves her to death and will want to follow her everywhere and idk what to do.