What do I do when my sister is so rebellious?

I have a VERY disobedient little sister, the age of 6 turning seven soon. She has a terrible temper and resorts to yelling, always. When she has to remind us of anything a second time, say: what time it was, and we happened to not hear the first time, she begins the yelling. She pouts and moans when she doesn't get what she wants, when trying to teach her when her answers are wrong, she stubbornly refuses to listen and denies the correct answer with a yell of "No!" I find it nearly impossible to get her to eat her food, drink her milk, etc. when she is reading her books, playing video games, or on the computer. I've threatened to take away her entertainment, tried to bribe her with things she likes, and every single time I need her to do something, I literally always have to tell her at least twice and tell her some more to get the job done if at all. She starts crying and screaming if I were to yell, or just ignores what I say completely and pretends she didn't hear if I say it nicely. When I try the strict voice, she just says, "You're not the boss!" She insists that our mother and father are the boss, but she refuses to listen to them either! What in the world do I do?!