dealing with picky eater !! what to do??

My niece who is 7 is a terribly picky eater. She is an only child and my sister has spoiled her greatly. Problem is my sister is living with me for couple of years till her school finishes and I'm taking care of her daughter. I cook and everything I put in table her daughter refuses to eat and start crying about it. My sister is becoming vocal about the fact that her daughter is an only child and is picky eater. I have two boys of my own and they have never said no to any food. I'm annoyed beyond words and don't know what to do. Any suggestions??