In chapter 83, in the panel that showed Grisha's and Zeke's faces, their facial structures looked exactly the same. So I think that they are brothers.
Ymir also has pointy ears, so either it doesn't, or my Ymir X Zeke fanfic will need some adjustments.It was too obvious to be honest not just the face but if you look at their titans transformation eren, grisha and zeke have goblin ears. witch clearly indicates a genetic conection.
I first assumed that Zeke is actually him, Grisha himself and that they would later explain how he survived, but now I also believe he must be Grisha's elder brother or his little brother.In chapter 83, in the panel that showed Grisha's and Zeke's faces, their facial structures looked exactly the same. So I think that they are brothers.