The Pool Guy - S7-E8
Trivia: When Jerry, Elaine, and Susan are at The Movie "Chunnel", Susan is annoyed while Jerry and Elaine are talking during The Movie. Listen to The Movie dialogue in the background of the scene. The Voice which yells "Everyone out of the chunnel!" is that of co-creator Larry David.
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.