Okay, on last nights episode, they showed Skippy's sister Arlene. I never knew Skippy had a sister. I thought he was an only child. Have they mentioned her before?

I think that's the only episode she every appears in. They never talk about her in any other episode either.

I also noticed while looking through the episode guide that Skippy
"discovers" in a later episode that he is adopted and Alex helps him meet his birth mother. However in the first episode that we meet Skippy he tells the Keatons that he is adopted, no secret there...