Chapter Eleven is the only name we know Cal's brother by. Here's how Cal describes him: "Pasty, and bald by the age of twenty-three, so that he makes a perfect homunculus" (2.7.8). Chapter Eleven doesn't really factor into Cal's life that much, which might be why we never learn his actual name. When he returns from college, he drives the family business into the ground. This could be the reason he is given a nickname that means bankruptcy, or it might just be an indicator of the emptiness of Cal and Chapter Eleven's relationship.
Anyway, we can't really think of much of anything that these two siblings have in common. We do have to briefly mention Chapter Eleven's girlfriend Meg Zemka, though. She does her best to make Chapter Eleven into the exact opposite of his father, and her attitude and politics could not be more opposite from Milton's if she tried. She introduces Chapter Eleven to acid, Marxism, and other progressive ideas and experiences. But by the time Chapter Eleven moves back home, she's not mentioned anymore. Meg must have just been a phase.