What to do when someone else's kid keeps getting smart with you?

My niece who is 12 years younger then me keeps getting smart with me. For example I tell her not to talk to me like that and she says 'I just did'. One of very many examples. She thinks she is my equivalent and she is not! No one lets me discipline her if I do my mom (her grandma) yells at me in front of her. She is here for the summer and I live here too. What can I do?