The Checks - S8-E7
Continuity mistake: When Jerry is showing George how he twirls the umbrella in his apartment, it has distinctive bright red and yellow stripes. He sets it on The Couch and then Kramer accidentally sits on it. It is now dark colored.
Chosen answer: George does mention his brother in the episode "The Parking Space". They actually really never mention anything that contradicts the fact he had a brother though, it just isn't mentioned.