What is unusual about Todd's brother?
I think that there is not much unusual about Todd's brother. This becomes the fundamental problem for Todd. His parents expect him to be a spitting image of his brother. Todd's brother excelled at school, at Welton, and represented the very best in conformity. Todd's brother earned excellent grades, graduated with the very best, and captained an outward appearance that stressed respect along with adherence to structure and discipline. Todd's parents expect him to be the exact same persona as his older brother. In this light, there is nothing unusual about him and more unusual that Todd would not want to be him.
Yet, here is where Todd's crucible, or test, lies. He begins to understand through Mr. Keating's teachings that he has a choice to make in terms of either being a clone of his brother than his parents expect him to be or he can "seize the day" and be himself, appreciating the talent and leadership potential that is inside him. It is here where Todd's brother is seen as unusual in that his purpose is to represent the establishment and the system. In Todd, the challenge lies between being a follower or the leader that Keating sees and that Todd recognizes is there, though buried deep.