Emily's older brother isn't anything like her. In fact, if they didn't look so much alike and share a last name, most people wouldn't know they're related. (This concept is loosely based on how my younger siblings and I look nothing alike but have similar personalities; Rob and Emily look alike but are completely different).

Rob is the equivalent of a junior at Riverwick -- Tammie and Emily would be considered freshmen if this were a modern day high school. He's seventeen years old, and incredibly intelligent. While Emily is infamous for causing trouble, Rob is praised for his mind. Like his sister, he has black curly hair, dark skin and blue eyes. He wears square glasses, is about 5'10" and is pretty skinny. 

From chapter fourteen:

I study him. He has curly black hair, olive skin, and blue eyes, like his younger sister, but just as Emily is bright and loud and spirited, Rob is quieter in everything he does. His hair is marginally more relaxed than Emily's, the eyes behind his glasses are more of a gray-blue than electric. His skin is a shade or two lighter than Emily's tan, and he moves with a patience she has probably never possessed.

Where Emily readily expresses her opinion and seeks the most obvious solution to a problem, Rob is soft-spoken and prefers the path of least resistance. If they were benders, Rob would be an Airbender and Emily would be a Firebender. I'm not kidding (I take my Avatar: the Last Airbender and Legend of Korra very seriously. Expect to see a lot of references and shoutouts. It deserves its own post, honestly). 

This being said, neither is better or worse than the other. They are both incredibly powerful and strong, but in different ways. Rob is much sneakier than Emily is. She has brute force and fear to support her, where he is logical, calculating and tends to fare better because of it. When the time comes, and the time will come, Rob is going to be tested. 

They all will.