Continuity mistake: The first time we see Jerry's dad, he's passively taking his wife's whine about The Jacket, remote control on his chest and hands crossed on his belly. Cut, and he is sitting with his arm by his sides. (00:01:00)
Continuity mistake: At the family dinner, Jerry's mom mentions horses for the first time. Reaction shot on the old lady saying "yeah, yeah", and cut to a wider angle; she is now all of a sudden leaning on her right arm. (00:06:50)
Continuity mistake: Jerry and Kramer are arguing about the bet when Kramer is having a glass of milk. The phone rings cutting them off, and Kramer's glass suddenly from his mouth goes to his hip. (00:10:25)
Continuity mistake: Jerry called his parents back to tell them about Myra's dead. When his dad at the end of the scene says "You know what this funeral's gonna wind up costing me?" Barney Martin is leaning his head against his hand all of a sudden. (00:12:50)
Continuity mistake: Jerry and his friends are eating, and Elaine opens defending The Pony Remark saying she was just having conversation. She gestures with both hands free, but then at the cut she asks Jerry about the game holding a teaspoon. (00:13:35)
Continuity mistake: During their casual talk about death and wasting your own life, George says he plans to continue wasting it. Cut, and both him and Elaine are in different poses than earlier, with Elaine licking the spoon and George with his fingertips by his mouth. In the majority of the conversation there are no such jumpcuts and continuity is kept. (00:14:45)
Continuity mistake: When Elaine tells George that Jerry must go to the funeral because he could have been the one who killed the old lady, Julia Louis-Dreyfus starts stirring the beverage. The spoon is gone at the cut, and at the following cut it is in her hand, which was free just a second before. Same later when George says that the dead don't care; she is holding it over the cup, but in the next shot it's suddenly inside the cup, hands off. (00:15:15)
Continuity mistake: When Jerry says that Manya's widow is moving to Phoenix, Julia Louis-Dreyfus ' teacup turns 180° between shots. (00:16:15)
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.