Board Thread:Manga/@comment-29919227-20160909220906/@comment-8458738-20160917005751

Manuel de la Fuente wrote: In ANSWERS, Isayama said this about Zeke:

"In the past, I heard the phrase: 'The rival who stands in the way of the main character is the form that the main character should never become, or the brother-like existence that he must surpass.' I want to make Zeke a character who is in that position."

In other words: "I must surpass my big bro, but I must not become like him".

Isayama's early design of Zeke (with less facial hair) also shows once again that he's a young man:

So you are saying Zeke is rather Grisha's brother, in essence, making him, Eren's Uncle. Which to be honest I find that more believable.