What should i do about my immature sister?

So my sister is 13 and she has required reading over the summer. My mom said i had to make sure she read today because we'd be gone for awhile to visit our whole entire family in china and we won't hav to time to read there. I told her to go read her required reading book which is ALSO my childhood favorite, the ruins of gorlan. I had to force her to read yesterday even though she said it was a good book. Today, she said she didn't feel like it and like everyday, i had to tempt her. I said if she did, i'd let her read her seventeen magazine. Instead, she ripped it out of my hands, went to the restroom and read till she was done with it. When she came out, she had a smug smile on her cause i couldn't do anything since my mom isn't here. Also, i no she won't yell at my sister cause she never does since she's SO special and SO sensitive and SO "disciplined". So wat should i do? I am seriously having trouble thinking of punishments. Don't say try to just talk to her or anything gentle. I've seen my mom do that to her and she just takes advantage of it and doesn't even think of what she does wrong.