HELP! My kid sister is a brat!!!

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Okay, so, my problem is my sister. She is twelve years old and I can't say that we have a terrible relationship. We can get along sometimes, but there are moments when I wish she wouldn't be such a brat. THis may seem little and I left it along, but this morning when I got up - she was having breakfast, the rule with us is that when we eat at the table we have to have a mat, but, she didn't have a mat. She ignored me when I told her to get one and she had her leg up so that she could rest her elbow on it. She isn't observing table manners at all and is making herself to be a complete slob. Telling any of my siblings to not eat like cows or pigs doesn't help because then they'll just make that animals sound and be even more annoying. I would like to take care of this myself, because Mom has a lot to do all ready. Is there anything that works for kids and might work for an older sister? Thanks.