Revealing mistake: When Carrie is talking about going to the fertility clinic, you can see the fridge behind Doug through the door. If you look closely just before the camera changes a boom shadow moves across the fridge. (00:01:35)
Revealing mistake: When the screen says "2 months later" the camera shakes. (00:21:20)
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.