[[WMG: Sondra Huxtable is a [[Series/BuffyTheVampireSlayer Key]].]]
That's why she doesn't exist in the first few episodes. The monks hadn't created her yet.

* She was a replacement for Denice the original/flawed Key. That would explain why Cliff only thought he had 4 children in the first episode. That is why they both look different from the other 3 Cosby kids.