She didn’t appear until Season 3 Episode 14. It was written off as a joke.
Cory told his mom and older brother, “You know, why can’t you guys learn from Morgan? She is a great sister. I mean, she stays in her room, you don’t hear from her, you don’t see her, and best of all, she stays out of my personal life.” When Morgan came downstairs to stick her nose in his business, he said, “Morgan, long time, no see!”
She replied, “Yeah, that was the longest timeout I’ve ever had!”