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My older sister is quick-witted.
Literal
Older-sister [topic-は] head [attributive-の] rotation [subject-が] good.
頭の回転がいい ('quick-witted, sharp') is a stock phrase for mental agility — literally 'the head's rotation is good.' The metaphor imagines the mind as a rotating engine, and speed of rotation = speed of thought. Related: 頭がいい ('smart'), 頭が切れる ('sharp-minded,' literally 'the head cuts'). A common compliment.