I plan to tell my sister to shut up.
What did she say that made you want to tell her that?
She keeps talking about how our parents love her more.
I understand why you're annoyed, but it's impolite to tell your sister to shut up just because she's saying mean things. You should just ignore her because she'll probably get bored and stop.
I don't like being nice.
Telling your sister to shutup is just going to make you come across as the bad guy. You are better off either distancing yourself from your sister, or talking to your parents about it.