Final answer:
The word 'alacrity' in the given context means cheerful readiness.
Step-by-step explanation:
The word 'alacrity' in the given context is best defined as cheerful readiness. In the sentence, the speaker expresses surprise at their sister's energy and enthusiasm in vacuuming and dusting the room. This indicates that the sister approached the task with willingness and eagerness, which aligns with the meaning of 'alacrity'.
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