Brother

In the past few months I have noticed that Justin does not call his brothers by name, he just says "brothers."

Justin: "Where is brother?"
Me: (knowing that he is talking about the brother that is not sitting next to him) "Which brother?"
Justin: "Brother!"

Justin: "Brother hit."
Me: "Which brother?"
Justin: "Brother!"

I know he knows his brothers name, but I rarely hear him say them.