AHHHHHH

okay so im babysitting again today a 2 year old girl a 4 year old boy and a 6 year old...the 2 year old is my niece and she is sick but the other 2 are my sister's friend's children and the 6 year old girl i am babysitting keeps lying to me and then i ask if she is lying and she proceeds to lie to me about lying and its very obvisous that she is lying....i know i should prolly tell her mom but i dont want to be like you child is a bad child....i dont know what to do...what should i do... HELP!!!!