how come in the first episodes of this show,sondra doesn't exist? i thought maybe she was in collage or something but then in one early episode claire says "isn't it great we have 4 wonderful children" why did they add her?

I think there are two reasons why Sondra was added:

1. Bill Cosby had 4 daughters and a son in real-life.
2. Lisa Bonet's character Denise didn't seem like she was the oldest child. Typically, the oldest child is supposed to be the most responsible child. But Denise wasn't, so they brought in a daughter who was at Princeton for the first couple of episodes.

Bill Cosby helped in the creation of the show and his original view of the show included Sondra being a college student. However, when the show began, as he put it "I had forgotten about her." So she had always been part of the show she was just mistakenly left out at the very beginning.