For an AmE answer, I would never use "cousin brother." If the gender wasn't obvious from context and pronouns, I would add "he's a guy" or something of that nature.
I rarely hear/say "maternal uncle," although that is the best fit. It is usually not important - plus if you say something like "Uncle Bob," like you do when you are a kid, unless you have two uncle Bobs, you're fine. Even then, I have two Aunt Debs, and the family just calls the married-in one by her maiden name.
However, "maternal grandfather" and the like are fairly more common, especially since grandparents are talked about without their name more than aunts and uncles. You only have four grandparents, and they kind of identify you, while your aunts and uncles are extended family. For example, "my paternal grandmother died last year, but my maternal grandmother is still alive," in response to a question such as "do you have any grandparents still alive."