Plot hole: When George and Jerry are discussing The Rye bread scheme, George says Susan is working late that night, so she won't be around when he sneaks The Rye into the house. But when George reels The Rye in with the fishing pole, Susan is standing there with her parents, wearing her coat. When did she get home? She couldn't have entered through the front door, because Jerry was there tossing The Rye up to George. And since it's a New York City brownstone, there's no backdoor that's accessible. There's no logical explanation for her getting into the house to see George reeling in The Rye.
Continuity mistake: When Jerry and Elaine are talking in The Apartment, after George and Kramer go see the horse, the G+W Motorwerkes magnet disappears from the fridge.
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.