We babysit our niece a lot and while that's fine, I'm having trouble disciplining her without thinking I'm coming across as evil. She's my DH's niece so I don't feel as comfortable telling her no or telling her to go to bed etc. She's a little spoilt, used to getting what she wants, eating pizza for dinner instead of proper food, etc. Thing is she is now with us two-three times a week. When I've been stricter before she's made comments like "I like auntie so and so, she's not strict with me" and I feel so embarrassed when she does. Basically I'm being outdone by a six year old! I want to get along with my in laws and sometimes I think oh God, will they think in being mean for telling her she needs to eat dinner before that chocolate? Because if I do she runs and tells them. My in laws (FIL and MIL) have told me I can be strict but whenever I am the child finds a way to get me back! 
I know I need to grow a pair but I don't want to be the evil b----! How do I manage her?