The Fix-Up - S3-E16
Character mistake: George calls Jerry to tell him about sex with Cynthia. He starts by saying he just got home, which Jerry repeats. A few lines later, George confirms they had sex at his own apartment. Why would he be getting home from his own apartment? (00:13:50)
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.