Easter egg: On Disc 4, go to the Episodes list. Select to the last episode: "The Finale". If you go down at the bottom there is a round object that is lighted up. Click on it, and then it shows you an alternative ending of the last episode of Seinfeld.
Easter egg: On the first disk of volume 1, season 1, if you go to the extras menu, press up until the jalapeno pepper on the fridge in the top of the screen is highlighted. Press it, and you'll see a short camera test of various sets with Jerry Seinfeld and Jason Alexander, with commentary by director Tom Cherones and Production Designer Tom Lazari.
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.