It sounds like your stepsister is at odds with just about everyone in her life, and also that you (understandably) don't have a ton of affection or respect for her.
Perhaps you could change your narrative from 'I've never taken sides in her disputes and she keeps picking fights with me' to 'I did choose sides by selecting a roommate who I enjoy rather than a chaos agent, and it is not a surprise and not personal that chaos'
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