What does it mean to dream about your step brother?

If your step brother is a real person and you dreamed about him, it means he's a guy you know and your wacky subconscious mind stuck him in a dream. It may "mean" something more than that, and it may not. "Wacky" is the Word to remember here. To the extent that dreams have any meaning, they tend to be related to your hopes and fears, and a person in a dream may represent someone else entirely... I've had many dreams in which person X was my mother or something, and in the dream it all seemed perfectly normal and logical until I woke up and realized "wait, I'm not even related to person X, and he's a guy anyway."

If you don't have a step brother it probably means you're intrigued by the concept.