Why does my family expect me to do all chores whilst step sister gets a free pass on all chores?

For e. g., if I don't do washing up one day but do it every other day suddenly I'm an asrehole. My Step Mum is a difficult individual to live with. Yet my Stepsister does not get accused of not doing chores. My Stepsister is not disabled.
Why does my family expect me to do all chores whilst step sister gets a free pass on all chores?
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