The fire happened after they were already in production, the changes were already made. Sheldon and his nephew were added for goofyness I guess, and I think with the housekeeper/nanny, they wanted more of a challenge to keep the secret. Zelda just wasn't paying that much attention to what was going on I guess...
I have the complete series on DVD (fan-made, bought it from a convention in NY many years ago), one thing I never could understand is why they changed the Sigmund costume as much as they did. It just did not have the same great kooky look of the original. All these years later, looking back, I wonder if they were trying to make Sigmund appear as getting "older" as the others stars were getting older as well?