My sister is such a brat!?

you need to find ways to get along with her

What I do with my overly spoilt younger siblings is take no nonsense and be firm and consistent with what you do.

Like I have a two year old sister Jumping on the draws gets her a smack on the bum and a firm telling off of if you do it again it will be a lot worst promise not to do it again and stay there till she promise she won't do it again.

Then I have twin brothers who recently turned nine and they get sharp knifes run up and down the stairs like they are plastic swords and they might get a graze or two. For them I normally sit one down in a room explain what they did was wrong explain why then give them a slap on the but and same for the other one.

Depends on what they do his old they are etc

Set an example, that's all you can do for now. She may be a brat, but you seeing this allows you to be better. When you get older, you may be able to appreciate this lesson, but for now you're just the better person. Rising above an immature situation doesn't gives one the benefit of satisfaction, immediately. But, in due time you'll see how much better you are than she, and she may learn from your example.