I hate my 7 year old cousin because she's so spoiled.
What makes you hate your cousin so much?
I hate how spoilt she is, and that she'll sometimes drink my drinks!
It's fine to not get along with a family member all of the time, especially when they're doing something that upsets you like drinking all of your drinks. You shouldn't judge them just because they're financially well-off though, and should instead try to instill values in her that will make her a better person.
I'm going to ignore my cousin which will show her that she is a bad person.
You shouldn't just ignore your cousin. Talk to her parents so they can fix the behavior when she is young.