Final answer:
The fifth sister's activity is not specifically indicated, but it can be inferred that she is likely playing chess with Kate.
Step-by-step explanation:
The fifth sister is not clearly specified in the question what she's doing. However, if you look at the question carefully, it can be inferred that the last sister is likely playing chess with Kate, as chess is a two-player game and all the other sisters' activities are accounted for.
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