I don't know if this has been already mention, but if Skippy was adopted, would that make his sister adopted too? Because his sister(forgot her name) looks just like him with the glasses and everything. Hmmmm.........I smell a writer boo boo.

I think that was explained early in the episode you are referring to. I could be wrong since it's been so long since I have actually seen the episode but I believe that Skippy's parents had his sister after he was adopted. The idea that Skippy's adopted parents couldn't have kids was never adressed so anything's possible. (Not explaining things properly is a great way to avoid serious continuity problems without backtracking or worrying about blatent screw-ups!)

L8