She Loves Me, She Loves Me Not - S1-E13
Corrected entry: In this episode, Topanga's sister comes to pick her up from Cory's house at the end. She is never mentioned or seen again.
She Loves Me, She Loves Me Not - S1-E13
Corrected entry: Topanga acts like she didn't know Eric was Cory's brother, but in later episodes, Topanga and Cory had known each other for practically their whole lives. How could Topanga have not known about Eric?
Correction: She didn't say "I don't know who Eric is" she probably just didn't recognize him. People can change a lot within a few years.
This correction isn't consistent with the way the scene and episode both play out. When Eric first shows up to talk to Cory and his classmates, Cory and Mr. Feeny both loudly announce his arrival by saying "Eric?" and "Mr. Matthews?" respectively, which elicits no immediate reaction from Topanga. Moments later when Mr. Feeny says "Please class, give Mr. Matthews your attention," a wide-eyed Topanga incredulously responds with "Matthews?", as if she didn't know Cory had a brother, despite Cory and Feeny both addressing Eric when he first arrived. This is further substantiated by Eric, who instead of responding with "Yeah Topanga, it's me Eric. Don't you recognize me?" says "Oh yeah, I'm Cory's older brother. I got the good hair." When Topanga shows up at their house the next day, Eric greets her with "Topanga, right?" and later says she's known him "for like 5 minutes," suggesting they had never met until the day before.
Correction: How is this a mistake, many guest stars in shows are never seen again. Maybe she moved out. Just because she isn't in any more story-lines doesn't mean she isn't around.
moviemogul
But in the later season when Topanga and her mom are talking about her and her father divorcing the mother implies that Topanga is their only child.