Continuity mistake: When George and Susan are arguing over dinner about her cousin stealing the name Seven, Susan holds her glass up to her mouth in one shot. In the very next shot, the glass is on the table.
Continuity mistake: When George is at Susan's cousins' house complaining about them stealing the name Seven, its bright Daylight out. You can tell because the sun is pouring in through the windows. The woman goes into labor and they all get into the car to go to the hospital. At this point it is nighttime. When they finally arrive at the hospital it is broad daylight again.
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.