Board Thread:Manga/@comment-27713352-20160909233230/@comment-27321453-20160913190752

Dave55811 wrote: The fact that Zeke greatly resembles Grisha is why I assume they're brothers. Zeke was surprised to see that Eren looks nothing like Grisha. Now that doesn't necessairly mean he's his half brother but it would hint at it with Zeke maybe thought to himself "I look greatly like Father, but HE doesn't?" about Eren.

Makes me conclude that Zeke wouldn't even believe that Eren is Grisha's son (he asked him but Eren didn't answer) had it not been for the Titan shifting marks on Eren's face while holding Bertolt hostage because Eren is the only Shifter inside the Walls who Zeke hasn't seen before.

Also, Eren and Levi are the only ones who saw Zeke's face close up (Hanji and Jean saw him briefly on top the Quad Titan) so it's interesting to see what Eren and Levi think upon the child in the photo's identity.