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Revealing mistake: Even after receiving the gift basket from the neighbors, the Heffernans are still awakened by their barking dog. When Carrie rushes downstairs Doug stops her at the front door, and the entire left window has been removed (for camera access) from its frame, so there's a large empty hole. (00:13:10)
Answer: Sara hasn't been mentioned in later episodes, and it suggested in later episodes that she never existed, and that Carrie is an only child. Kevin James stated that the producers couldn't think of an good plots to use with Sara, so they left her out.