Can I tell my sister that she's a spoiled brat?
why do you think your sister is a spoiled brat?
Because she always gets what she wants.
It's ok if your sister gets whatever she wants. You dont need to call her spoiled brat for that.
I should show my sister who the parent is.
You're not her parent so you have nothing to show her, just allow your sister to live her life and you live yours. People would think you're being very petty for this.