What should I do when my sisters girl freind is a ass?

Suddenly my sister brought this girl into our house, Let's call her "mia". She is all kind and everything, till they start dating, She turned into this great monster! Everyone loves her, she's perfect just because she cleans the house, But she hurts me in so many ways physicaly and mentialy. BUT nooooo, no one cares about the youngest! Am i the ass?

What should I do when my sisters girl freind is a ass?
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