Continuity mistake: When Carrie finds Doug in the kitchen in the middle of the night, her arms are crossed with her hands tucked into the sleeves. After she asks him about his pay check from IPS, the shot changes and her hands are now on her hips. (00:10:50)
Continuity mistake: When Doug is talking to Carrie in the kitchen about his ideas for the sandwich shop, lights reflecting in the living room windows behind Doug turn off and then back on when the shot changes between Doug and Carrie. (00:11:00 - 00:11:35)
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.