The Bottle Deposit (1) - S7-E21
Character mistake: When Jerry tells Kramer and Newman that they left their groceries under the hood of his car, he comments, "The AAA guy said I was this close to sucking a muffin down the carburetor." Jerry's car is a Saab 9000, which are fuel injected.
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.