it is a phase....
She is viewing you as her buddy or friend, not as her mommy's sister. She is testing...and it will stop when she realizes you have more authority and higher standing. It just takes toddlers a long time and if you try to set her straight, it will cause resentment. Give her time and let her come to you. If she is to the point of rude, I would hope her mother would react appropriately to let her child know she is behaving rudely.
My nephew was once very nasty. And I hate to say it, but his father let it happen so i began to view him as a mean nasty little boy. When I came to visit, it was to visit the adults....he ignored me. And well being ignored was better than being hit or pinched. Well one day it changed, he was suddenly in line for a big hug and must've learned how to respect a guest if he wanted their attention.
Good luck, just give her time.