annoying "step-mother"

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Ok, so my dad and his "girlfriend" have been dating for over 6 years now since my parents separation, and this "girlfriend" (whom recently turned into fiance) is driving me insane. She's a complete opposite from my biological mother and everything she does angers me. I have 4 other siblings who all feel the same way, we hate her, but she's not a bad person, she seems to be imposing herself as our mother, and we don't want to stand for it. What should I do? I'm sure others have this case scenario or something similar!