Matilda's brother is, well, barely there. We meet him, and then we forget him. In fact, we probably forget about him in the same type of way that Matilda's parents forget about her. He's ordinary. Next to his genius sister, he looks really dull. His parents favor him—and that's one of the ways we know what great judges of character they are.
That's not to say that we should rag on Mike. He's not mean to his sister. In fact, he seems to be the only member of their nuclear family who's sad to see her stay behind and not go to Spain. When it comes to intelligence, though, he seems to take after his dad, only he's even slower at doing calculations. Mike's supposed to enter the family business someday and, unfortunately, it's hard to imagine many cars getting sold with him at the front of the house.
Mike is a true Wormwood, and in that way, he represents what Matilda is most definitely not.