I'm going to cancel plans with my sister
What problem came up that you had to cancel?
She is being so rude to me and treating me like I'm beneath her.
It sucks she's rude to you, but plans are plans and you need to be the mature bigger person and stick to them. Canceling these plans is breaking your trust further with her, and you can't risk that when you should mend your relationship during these plans.
I'm going to do it. I don't care for my family.
I understand why you would not want to be around your sister, however family is the most important thing we have in life so rather than burning bridges with them, you should communicate your issues.